UPSC Prelims 2020 · CSAT
Question 7: Basic Numeracy
How many zeroes are there at the end of the following product? 1 × 5 × 10 × 15 × 20 × 25 × 30 × 35 × 40 × 45 × 50 × 55 × 60
AnswerUPSC’s answer is (a).
Trailing zeroes come from pairs of 2 and 5. Count the 5s: 5, 10, 15, 20, 30, 35, 40, 45, 55 and 60 give one each (10), and 25 and 50 give two each (4), so fourteen 5s. Count the 2s: 10 (1), 20 (2), 30 (1), 40 (3), 50 (1), 60 (2) give ten. The number of zeroes equals the smaller count, ten.
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) Right: only ten factors of 2 exist, so only ten pairs with 5 can be made.
- (b) Twelve is not the count of either factor.
- (c) Fourteen is the number of 5s, but the 2s limit the pairs to ten.
- (d) Fifteen matches neither count.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2020, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.