CSAT 2020

Q. 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?

(a) 28
(b) 29
(c) 30
(d) 31
Correct Answer: (b) 29

Question from UPSC Prelims 2020 CSAT Paper

Explanation :

Frog tries to come out of well

With each jump, the frog effectively moves up by 30 cm – 15 cm = 15 cm. The total distance the frog needs to cover to get out of the well is 4.5 m or 450 cm.

If we divide the total distance by the effective distance covered in each jump we get: 450 cm / 15 cm/jump = 30 jumps

However, this calculation assumes that the frog slides down after its last jump which is not the case since it would have already reached the top of the well.

So we need to subtract one jump from our previous calculation to account for this and get a final answer of 29 jumps.

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