UPSC Prelims 2019 · CSAT
Question 36: Basic Numeracy
X, Y and Z are three contestants in a race of 1000 m. Assume that all run with different uniform speeds. X gives Y a start of 40 m and X gives Z a start of 64 m. If Y and Z were to compete in a race of 1000 m, how many metres start will Y give to Z ?
AnswerUPSC’s answer is (b).
When X runs 1000 m, Y runs 960 m and Z runs 936 m. So Y's speed to Z's speed is 960 : 936 = 40 : 39. In a 1000 m race, when Y covers 1000 m, Z covers 1000 × 39/40 = 975 m. So Y must give Z a start of 25 m. Do not subtract 64 - 40 directly, because those starts are measured against different runners.
How you could have got this
No shortcut here: in a “how many” question, ruling out options doesn’t help. Every statement has to be judged.
Which options go is worked out from UPSC’s key and the options, not by a model.
Why each option is right or wrong
- (a) Wrong: 20 does not follow from the 40 : 39 speed ratio.
- (b) Right: Z runs 975 m while Y runs 1000 m.
- (c) Wrong: 30 does not match 40 : 39.
- (d) Wrong: 35 does not match 40 : 39.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2019, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.