UPSC 2020 Prelims PYQs

Question 1

A frog tries to come out of a dried well 4.5 m deep with slippery walls. Every time the frog jumps 30 cm, slides down 15 cm. What is the number of jumps required for the frog to come out of the well?

Accepted Answer

Correct: B. 29. Each jump gains 30 cm but slides back 15 cm, so net gain before the final jump is 15 cm. The well is 4.5 m = 450 cm deep. After 28 jumps with sliding, height gained = 28 x 15 = 420 cm. On the 29th jump, the frog reaches 450 cm and comes out before sliding back. Hence 29 jumps are req

Question 2

A vessel full of water weighs 40 kg. If it is one-third filled, its weight becomes 20 kg. What is the weight of the empty vessel?

Accepted Answer

Correct: A. The keyed answer is A: 10 kg. List the given conditions, convert them into a table or symbolic form, and test the options against all conditions together. Option A is retained because it is consistent with the complete condition set; the other options fail at least one stated condition o

Question 3

If 1 litre of water weighs 1 kg, then how many cubic millimetres of water will weigh 0.1 gm?

Accepted Answer

Correct: C. 100. 1 litre of water weighs 1 kg = 1000 g. Therefore 0.1 g of water has volume 0.1 ml. Since 1 ml = 1000 cubic millimetres, 0.1 ml = 100 cubic millimetres.

Question 4

A digit n > 3 is divisible by 3 but not divisible by 6. Which one of the following is divisible by 4?

Accepted Answer

Correct: D. 3n + 1. Since n is a digit greater than 3, divisible by 3 but not by 6, n must be 9. Then 2n=18, 3n=27, 2n+4=22 and 3n+1=28. Only 28 is divisible by 4.

Question 5

Which one of the following will have minimum change in its value if 5 is added to both numerator and the denominator of the fractions 2/3, 3/4, 4/5 and 5/6?

Accepted Answer

Correct: D. The keyed answer is D: 5/6. List the given conditions, convert them into a table or symbolic form, and test the options against all conditions together. Option D is retained because it is consistent with the complete condition set; the other options fail at least one stated condition or

Question 6

How many pairs of natural numbers are there such that the difference of whose squares is 63?

Accepted Answer

Correct: A. 3. Difference of squares: a^2 - b^2 = (a-b)(a+b) = 63. Factor pairs of 63 with the same parity are (1,63), (3,21), and (7,9). These give natural-number pairs (32,31), (12,9), and (8,1). Hence 3 pairs are possible.

Question 7

A man takes half time in rowing a certain distance downstream than upstream. What is the ratio of the speed in still water to the speed of current?

Accepted Answer

Correct: D. The keyed answer is D: 3 : 1. Treat the answer choices as testable values. Substitute each option into the given conditions and reject any value that violates even one condition. Option D is the one that remains consistent with the complete set of conditions.

Question 8

Consider the following statements 1. The minimum number of points of intersection of a square and a circle is 2. 2. The maximum number of points of intersection of a square and a circle is 8. Which of the above statements is/are correct?

Accepted Answer

Correct: B. 2 only. A circle may touch a square at one point, or may not intersect it at all, so the minimum number of intersection points is not necessarily 2. The maximum occurs when the circle cuts each of the four sides at two points, giving 8 intersections. Hence only statement 2 is correct.

Question 9

A car travels from a place X to place Y at an average speed of km/hr, from Y to X at an average speed of 2v km/hr, again from X to Y at an average speed of 3v km/hr and again from Y to X at an average speed of 4v km/hr. Then the average speed of the car for the entire journey

Accepted Answer

Correct: B. lies between v and 2v. The four equal-distance trips have speeds v, 2v, 3v and 4v. Average speed for equal distances is total distance divided by total time: 4 / (1/v + 1/2v + 1/3v + 1/4v) = 4v/(1 + 1/2 + 1/3 + 1/4) = 48v/25 = 1.92v. This lies between v and 2v.

Question 10

A person X can complete 20% of work in 8 days and another person Y can complete 25% of the same work in 6 days. If they work together, in how many days will 40% of the work be completed?

Accepted Answer

Correct: A. 6. X completes 20% in 8 days, so X's rate = 2.5% per day. Y completes 25% in 6 days, so Y's rate = 25/6% per day. Together, rate = 2.5 + 25/6 = 5/2 + 25/6 = 40/6 = 20/3% per day. Time for 40% work = 40/(20/3) = 6 days.

UPSC 2020 Question 1: CSAT

UPSC 2020 Question 2: CSAT

UPSC 2020 Question 3: CSAT

UPSC 2020 Question 4: CSAT

UPSC 2020 Question 5: CSAT

UPSC 2020 Question 6: CSAT

UPSC 2020 Question 7: CSAT

UPSC 2020 Question 8: CSAT

UPSC 2020 Question 9: CSAT

UPSC 2020 Question 10: CSAT

UPSC 2020 Prelims Questions