UPSC Prelims 2020 · CSAT

Question 80: Basic Numeracy

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?

a28

b29

c30

d31

AnswerUPSC’s answer is (b).

Each jump nets 30 - 15 = 15 cm, but the frog does not slide back once it is out. It escapes when a jump takes it to 450 cm, so it must be at 420 cm or more before the last jump. After 28 jumps and slides it is at 28 × 15 = 420 cm, and the 29th jump reaches 450 cm. So 29 jumps.

Why each option is right or wrong

(a) After 28 jumps it is at 420 cm after sliding; one more jump is needed.
(b) Right: the 29th jump clears the top without a slide.
(c) 30 jumps counts a jump that is not needed.
(d) 31 jumps counts two jumps that are not needed.

Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2020, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.