UPSC Prelims 2017 · CSAT
Question 16: Basic Numeracy
There are three pillars X, Y and Z of different heights. Three spiders A, B and C start to climb on these pillars simultaneously. In one chance, A climbs on X by 6 cm but slips down 1 cm. B climbs on Y by 7 cm but slips down 3 cm. C climbs on Z by 6·5 cm but slips down 2 cm. If each of them requires 40 chances to reach the top of the pillars, what is the height of the shortest pillar?
AnswerUPSC’s answer is (b).
Each chance, a spider climbs and slips back, but on the final chance it reaches the top and does not slip. So height = 39 net gains + the last climb. A: net 5, so 39 x 5 + 6 = 201 cm. B: net 4, so 39 x 4 + 7 = 163 cm. C: net 4.5, so 39 x 4.5 + 6.5 = 182 cm. The shortest pillar is Y at 163 cm. The common slip is multiplying all 40 chances by the net gain.
Why each option is right or wrong
- (a) 161 cm does not match any pillar from the working.
- (b) Correct: pillar Y is 39 x 4 + 7 = 163 cm.
- (c) 182 cm is pillar Z, which is taller than Y.
- (d) 210 cm does not match any pillar; pillar X is 201 cm.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2017, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.