UPSC Prelims 2019 · CSAT
Question 11: Basic Numeracy
A and B are two heavy steel blocks. If B is placed on the top of A, the weight increases by 60%. How much weight will reduce with respect to the total weight of A and B, if B is removed from the top of A ?
AnswerUPSC’s answer is (d).
Putting B on A raises the weight by 60% of A's weight, so B = 0.6 A and the combined weight is 1.6 A. Removing B takes away 0.6 out of 1.6, which is 3/8 = 37.5%. The trap is the base: 60% is B as a share of A alone, but the question asks for the reduction relative to the combined weight of A and B.
Why each option is right or wrong
- (a) Wrong: 60% is B relative to A, not to the total.
- (b) Wrong: no step of the working gives 45.5%.
- (c) Wrong: 40% does not equal 0.6 / 1.6.
- (d) Right: 0.6 / 1.6 = 37.5%.
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.