UPSC Prelims PYQs with explanations
Real questions from official papers, mapped to syllabus topics. Attempt each one; the answer and the reason appear instantly.
There are large number of silver coins weighing 2 gm, 5 gm, 10 gm, 25 gm, 50 gm each. Consider the following statements :
- 1. To buy 78 gm of coins one must buy at least 7 coins.
- 2. To weigh 78 gm using these coins one can use less than 7 coins.
Which of the statements given above is/are correct ?
Answer (c) & explanation
Both statements are correct (c). Step 1: buying 78 g: 50 + 10 + 10 + 2 + 2 + 2 + 2 = 78 with 7 coins. 50 + 25 leaves 3 g, which no coin can make. Other mixes need more coins. So at least 7 are needed (1 true). Step 2: weighing allows coins on both pans. Put 50 + 25 + 5 on one pan and 2 on the other. That measures 80 − 2 = 78 g with only 4 coins (2 true). Step 3: so options (a), (b) and (d) fail.
Tip: "buy" means adding coins only; "weigh" lets you subtract by using the other pan. The wording changes the answer.
'buy' and 'weigh' are different. On a balance, a coin can also subtract.
Answer as per the official UPSC answer key.
Consider the following statements :
Statement-I :
Interest income from the deposits in Infrastructure Investment Trusts (InvITs) distributed to their investors is exempted from tax, but the dividend is taxable.
Statement-II :
InvITs are recognized as borrowers under the 'Securitization and Reconstruction of Financial Assets and Enforcement of Security Interest Act, 2002'.
Which one of the following is correct in respect of the above statements?
Answer (d) & explanation
Statement I is wrong but Statement II is correct (d). Statement I is wrong because it is reversed: interest income that an InvIT passes on to investors is taxable in their hands. Dividends are usually exempt, as long as the project company has not opted for the lower corporate tax regime. Statement II is right: the Finance Act, 2021 made InvITs and REITs borrowers under the SARFAESI Act, 2002, so lenders can enforce security against them, which makes lending to them safer.
Notice what an InvIT is: like a mutual fund for infrastructure. It pools investors' money into income-earning assets such as toll roads, power lines and pipelines, and passes on most of the income. SEBI regulates both InvITs and REITs. The government uses InvITs for asset monetisation.
Statement I swaps interest and dividend. One swap is enough to make it wrong.
Answer as per the official UPSC answer key.
Consider the following actions :
- 1. Detection of car crash/collision which results in the deployment of airbags almost instantaneously
- 2. Detection of accidental free fall of a laptop towards the ground which results in the immediate turning off of the hard drive
- 3. Detection of the tilt of the smartphone which results in the rotation of display between portrait and landscape mode
In how many of the above actions is the function of accelerometer required?
Answer (c) & explanation
An accelerometer is needed in all three (c). An accelerometer, usually a tiny MEMS chip, senses changes in speed and the direction of gravity. Action 1 needs it: in a crash, it detects the sudden deceleration and fires the airbags within milliseconds. Action 2 needs it: when a laptop is in free fall, the sensor feels near-zero gravity and parks the hard drive heads to avoid damage. Action 3 needs it: it measures the tilt of a phone against gravity to switch the screen between portrait and landscape.
Notice its partners: a gyroscope measures rotation, and a magnetometer works like a compass. Together they power step counters, games and navigation when GPS drops. Smartwatches also use them for fall detection.
Answer as per the official UPSC answer key.
Consider the following statements :
Statement-I :
Recently, the United States of America (USA) and the European Union (EU) have launched the 'Trade and Technology Council'.
Statement-II :
The USA and the EU claim that through this they are trying to bring technological progress and physical productivity under their control.
Which one of the following is correct in respect of the above statements?
Answer (c) & explanation
Statement I is correct but Statement II is wrong (c). Statement I is right: the EU–US Trade and Technology Council was launched in June 2021 to coordinate on trade, supply chains, semiconductors and technology standards, based on shared democratic values. Statement II is wrong: its stated goals are cooperation and common rules, such as on artificial intelligence, not "control" over technological progress and productivity.
Notice that India and the EU set up their own Trade and Technology Council, announced in 2022 and launched in 2023, the EU's second after the USA and India's first such body with any partner. It has working groups on strategic technologies, clean energy and resilient supply chains. India and the EU also concluded their long-pending free trade talks.
Answer as per the official UPSC answer key.
A 3-digit number ABC, on multiplication with D gives 37DD where A, B, C and D are different non-zero digits. What is the value of A + B + C?
Answer (a) & explanation
A + B + C is 18 (a). Step 1: 37DD = 3700 + 11D. Since ABC × D = 37DD, D must divide 3700. So D is 1, 2, 4 or 5. Step 2: D = 1 or 2 makes ABC a 4-digit number. D = 5 gives 3755 ÷ 5 = 751, which repeats the digit 5. Step 3: D = 4: 3744 ÷ 4 = 936. All digits differ. A + B + C = 9 + 3 + 6 = 18. Option (d) is wrong: the data fix the answer.
Tip: split a coded number into a fixed part and a part with the unknown. Divisibility then narrows the choices quickly.
Answer as per the official UPSC answer key.
What is the number of selections of 10 consecutive things out of 12 things in a circle taken in the clockwise direction ?
Answer (c) & explanation
There are 12 selections (c). Step 1: in a circle, a run of consecutive things is fixed by where it starts. Step 2: any of the 12 things can be the starting point. Going clockwise, each start gives a different run of 10. Step 3: so there are 12 selections. Option (a), 3, applies to a straight line of 12 (12 − 10 + 1). Option (d), 66, counts any 10 things, not consecutive ones.
Tip: in a line, runs of k from n = n − k + 1. In a circle (with k less than n), runs of k = n.
66 = 12C10 counts all selections, consecutive or not.
Answer as per the official UPSC answer key.
The letters of the word "INCOMPREHENSIBILITIES" are arranged alphabetically in reverse order. How many positions of the letter/letters will remain unchanged ?
Answer (c) & explanation
Two positions stay the same (c). Step 1: the 21 letters in reverse alphabetical order are: T S S R P O N N M L I I I I I H E E E C B. Step 2: the original word is I N C O M P R E H E N S I B I L I T I E S. Step 3: compare place by place. Only places 13 and 15 match, both with the letter I. So two positions stay unchanged. Options (a), (b) and (d) miscount.
Tip: write both orders in two rows, one under the other. Count the letters carefully first; repeated letters like I (5 times) decide the answer.
write both rows one above the other and compare letter by letter.
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Answer as per the official UPSC answer key.
Consider the following statements :
Statement-I :
- In India, prisons are managed by State Governments with their own rules and regulations for the day-to-day administration of prisons.
Statement-II :
- In India, prisons are governed by the Prisons Act, 1894 which expressly kept the subject of prisons in the control of Provincial Governments.
Which one of the following is correct in respect of the above statements?
Answer (a) & explanation
Both statements are correct and II explains I (a). Statement I is right: prisons are Entry 4 of the State List, so each State runs them under its own jail manual and rules. Statement II is right: the colonial Prisons Act, 1894 still applies and treats prisons as a provincial matter. UPSC accepted that this old arrangement explains why States run prisons today.
Notice the reform push. In 2023, the Home Ministry issued a Model Prisons Act, to replace the 1894 Act, with ideas like open jails, video trials and help for rehabilitation. India's prisons are overcrowded, and most inmates are undertrials, people not yet convicted. The Supreme Court has repeatedly urged bail for poor undertrials and the end of caste-based work in jails (2024).
Answer as per the official UPSC answer key.
Consider the following statements :
- 1. In a seismograph, P waves are recorded earlier than S waves.
- 2. In P waves, the individual particles vibrate to and fro in the direction of wave propagation whereas in S waves, the particles vibrate up and down at right angles to the direction of wave propagation.
Which of the statements given above is/are correct?
Answer (c) & explanation
Both statements are correct (c). Statement 1 is right: P (primary) waves are the fastest seismic waves, so a seismograph records them first, before S (secondary) waves. Statement 2 is right: P waves are longitudinal: particles vibrate back and forth along the direction of travel, like sound. S waves are transverse: particles move up and down, at right angles to the direction of travel.
Notice what they reveal about the Earth. P waves pass through solids, liquids and gases, but S waves cannot pass through liquids. Since S waves do not cross the outer core, creating a shadow zone, we know the outer core is liquid. Surface waves come last and cause the most damage. The gap between P and S arrival tells the distance to the epicentre.
Answer as per the official UPSC answer key.
In the recent years Chad, Guinea, Mali and Sudan caught the international attention for which one of the following reasons common to all of them?
Answer (d) & explanation
All four saw successful coups (d). Mali had coups in 2020 and 2021. Chad saw a military takeover in 2021 after President Idriss Déby was killed. Guinea's army ousted President Condé in 2021. Sudan's army seized power in October 2021, and the army's later split with the RSF led to civil war in 2023. Option (a) is wrong: no shared rare earth find. Option (b) is wrong: China's only acknowledged overseas base is in Djibouti. Option (c) is wrong: desertification affects the Sahel, but it is not what links these four.
Notice the "coup belt" across Africa: Burkina Faso (2022), Niger and Gabon (2023) followed. Mali, Burkina Faso and Niger left ECOWAS and formed the Alliance of Sahel States.
Answer as per the official UPSC answer key.
A number N is formed by writing 9 for 99 times. What is the remainder if N is divided by 13 ?
Answer (a) & explanation
The remainder is 11 (a). Step 1: 999999 = 13 × 76923. So every block of six 9s is a multiple of 13. Step 2: 99 = 6 × 16 + 3. After 16 blocks, only 999 is left over. Step 3: 999 = 13 × 76 + 11. So the remainder is 11. Options (b), (c) and (d) do not match.
Tip: 999999 (six 9s) is divisible by 7, 11, 13 and 37. Use it to cut long strings of 9s into simple blocks.
Answer as per the official UPSC answer key.
Consider a 3-digit number.
Question : What is the number ?
Statement-1: The sum of the digits of the number is equal to the product of the digits.
Statement-2: The number is divisible by the sum of the digits of the number.
Which one of the following is correct in respect of the above Question and the Statements ?
Answer (d) & explanation
Even both statements are not enough (d). Step 1: Statement-1: the digit sum equals the digit product. For three digits this works for 1, 2 and 3 (1 + 2 + 3 = 1 × 2 × 3 = 6), in any order: six numbers. Step 2: Statement-2: the number must be divisible by the digit sum, 6. So it must be even. Step 3: both 132 and 312 survive. Two answers remain, so the number is not fixed. Options (a), (b) and (c) fail.
Tip: in "what is the number" questions, keep testing until only one number is left. Two survivors mean insufficient data.
Answer as per the official UPSC answer key.
Consider the following statements :
Statement-I : India, despite having uranium deposits, depends on coal for most of its electricity production.
Statement-II : Uranium, enriched to the extent of at least 60%, is required for the production of electricity.
Which one of the following is correct in respect of the above statements?
Answer (c) & explanation
Statement I is correct but Statement II is wrong (c). Statement I is right: India has uranium, though mostly low-grade, yet coal gives about 70–75% of its electricity, while nuclear gives only about 3%. Statement II is wrong: power reactors do not need 60% enrichment. India's PHWRs use natural uranium, and light water reactors use uranium enriched to only 3–5%. About 90% enrichment is weapons-grade.
Notice why nuclear stays small: low-grade ore, limited fuel imports and high costs. India plans 100 GW of nuclear power by 2047, with Bharat Small Reactors and private participation. Its three-stage programme aims to use its large thorium reserves in the end.
Answer as per the official UPSC answer key.
With reference to ancient South India, Korkai, Poompuhar and Muchiri were well known as
Answer (b) & explanation
Korkai, Poompuhar and Muchiri were ports (b). These were the three main Sangam-age ports. Korkai was the Pandya port, famous for pearls. Poompuhar, or Kaveripattinam, at the mouth of the Kaveri, was the Chola port, described in the Silappatikaram. Muchiri, or Muziris, on the Periyar, was the Chera port that traded pepper with Rome. Option (a) is wrong: the capitals were Madurai (Pandya), Uraiyur (Chola) and Vanji (Chera). Options (c) and (d) do not fit.
Notice the Muziris Papyrus, a Roman loan contract for a cargo from Muziris. Excavations at Pattanam in Kerala are linked to Muziris, and Arikamedu near Puducherry was another Indo-Roman trading site.
Answer as per the official UPSC answer key.
Consider the following statements :
Statement-I : According to the United Nations’ World Water Development Report, 2022, India extracts more than a quarter of the world’s groundwater withdrawal each year.
Statement-II : India needs to extract more than a quarter of the world’s groundwater each year to satisfy the drinking water and sanitation needs of almost 18% of world’s population living in its territory.
Which one of the following is correct in respect of the above statements?
Answer (c) & explanation
Statement I is correct but Statement II is wrong (c). Statement I is right: the UN World Water Development Report 2022, on groundwater, says India is the world's largest groundwater user, taking about a quarter of global withdrawals, more than China and the USA together. Statement II is wrong: about 90% of India's groundwater goes to irrigation, not to drinking water and sanitation, so it gives the wrong reason. India also has about 18% of the world's people, but that is not why it pumps so much.
Notice the cause: cheap or free electricity for farm pumps and water-hungry crops like paddy and sugarcane in dry regions such as Punjab. The Atal Bhujal Yojana and the CGWA's rules try to curb over-extraction.
Answer as per the official UPSC answer key.
There are four letters and four envelopes and exactly one letter is to be put in exactly one envelope with the correct address. If the letters are randomly inserted into the envelopes, then consider the following statements :
- 1. It is possible that exactly one letter goes into an incorrect envelope.
- 2. There are only six ways in which only two letters can go into the correct envelopes.
Which of the statements given above is/are correct?
Answer (b) & explanation
Only statement 2 is correct (b). Step 1: if three letters are in the right envelopes, only one envelope is left, and it is the right one for the fourth letter. So exactly one wrong letter is impossible. Statement 1 is false. Step 2: for exactly two right letters, choose which two are right: AB, AC, AD, BC, BD, CD = 6 ways. Step 3: the other two must both be wrong, so they swap, in only 1 way. Total = 6 × 1 = 6. Statement 2 is true. Options (a), (c) and (d) fail.
Tip: in matching problems, "exactly one wrong" can never happen. A wrong letter always pushes at least one other letter out of place.
Answer as per the official UPSC answer key.
Consider the following statements :
- 1. A is older than B.
- 2. C and D are of the same age.
- 3. E is the youngest.
- 4. F is younger than D.
- 5. F is older than A.
How many statements given above are required to determine the oldest person/persons ?
Answer (d) & explanation
All five statements are needed (d). Step 1: put them in a chain: C = D (2), D > F (4), F > A (5), A > B (1), and E is the youngest (3). So C = D > F > A > B, and E is last. Step 2: to call C and D the oldest, we must show they beat everyone. B is beaten only through statement 1. E is placed only by statement 3. Step 3: C is linked to D only through statement 2. Drop any one and someone's position is unknown. So all five are needed. Options (a), (b) and (c) fail.
Tip: to name the oldest, every other person must be placed below. Check each person is covered by at least one statement.
Answer as per the official UPSC answer key.
Consider the following :
- I. A + B means A is neither smaller nor equal to B.
- II. A − B means A is not greater than B.
- III. A × B means A is not smaller than B.
- IV. A ÷ B means A is neither greater nor equal to B.
- V. A ± B means A is neither smaller nor greater than B.
Statement : P × Q, P − T, T ÷ R, R ± S
Conclusion-1 : Q ± T
Conclusion-2 : S + Q
Which one of the following is correct in respect of the above Statement and the Conclusions ?
Answer (b) & explanation
Only Conclusion-2 follows (b). Step 1: decode the symbols: + means >, − means ≤, × means ≥, ÷ means < and ± means =. Step 2: the statement becomes P ≥ Q, P ≤ T, T < R, R = S. So S = R > T ≥ P ≥ Q. Step 3: Conclusion-1 (Q = T) need not be true; T could be bigger. Conclusion-2 (S > Q) is true, since S > T ≥ Q. So options (a), (c) and (d) fail.
Tip: decode every symbol first, then write one chain. A single strict ">" in the chain makes the end-to-end link strict.
Answer as per the official UPSC answer key.
Consider the following statements :
Statement-I :
In the post-pandemic recent past, many Central Banks worldwide had carried out interest rate hikes.
Statement-II :
Central Banks generally assume that they have the ability to counteract the rising consumer prices via monetary policy means.
Which one of the following is correct in respect of the above statements?
Answer (a) & explanation
Both statements are correct and II explains I (a). Statement I is right: after 2021, prices rose sharply worldwide because of supply chains broken by the pandemic and the Ukraine war. The US Federal Reserve, the European Central Bank, the Bank of England and the RBI all raised policy rates; the RBI raised the repo rate by 250 basis points in 2022–23, to 6.5%. Statement II is right and is the reason: central banks believe that dearer credit cools spending and demand, and so brings inflation down.
Notice the limit: monetary policy works on demand, but much of that inflation came from supply shocks, such as fuel and food. With inflation falling, the RBI began cutting rates again in 2025.
Answer as per the official UPSC answer key.
With reference to the role of biofilters in Recirculating Aquaculture System, consider the following statements :
- 1. Biofilters provide waste treatment by removing uneaten fish feed.
- 2. Biofilters convert ammonia present in fish waste to nitrate.
- 3. Biofilters increase phosphorus as nutrient for fish in water.
How many of the statements given above are correct?
Answer (b) & explanation
Two statements are correct (b). A Recirculating Aquaculture System cleans and reuses the same water, so it needs far less water and land. Statement 1 is right: the biofilter treats wastes from fish droppings and uneaten feed, after solids are removed. Statement 2 is right: its bacteria turn toxic ammonia from fish waste into nitrite and then into much less harmful nitrate; this is nitrification. Statement 3 is wrong: biofilters do not raise phosphorus; fish need clean water, and extra phosphorus only feeds unwanted algae.
Notice the link with aquaponics, where the nitrate-rich water feeds plants grown alongside the fish. India's PM Matsya Sampada Yojana (2020) funds RAS and biofloc units. India is the world's second-largest fish producer.
Answer as per the official UPSC answer key.