UPSC Prelims 1995 · General Studies
Question 138: Basic Numeracy
A rectangular sump of dimensions 6 m × 5 m × 4 m is to be built by using bricks to make the outer dimension 6.2 m × 5.2 m × 4.2 m. Approximately how many bricks of size 20 cm × 10 cm × 5 cm are required to build the sump for storing water?
AnswerThe answer is (c).
The bricks fill the volume between the outer and inner boxes: outer 6.2 x 5.2 x 4.2 = 135.4 cubic metres, inner 6 x 5 x 4 = 120 cubic metres, a difference of about 15.4 cubic metres. One brick is 20 x 10 x 5 cm = 1000 cubic centimetres = 0.001 cubic metre. So about 15.4 / 0.001, or roughly 15,400 bricks, is nearest to 15,000.
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.
This follows an unofficial answer: UPSC has not published its key for this year, and if that answer is wrong, so is this path.
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) 1500 is ten times too few.
- (b) 3000 is five times too few.
- (c) Right: about 15,000.
- (d) 30,000 is double the volume needed.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 1995, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.