UPSC Prelims 2016 · CSAT
Question 63: Basic Numeracy
How many numbers are there between 100 and 300 which either begin with or end with 2 ?
AnswerUPSC’s answer is (a).
Numbers strictly between 100 and 300 that start with 2 are 200 to 299: 100 numbers. Numbers ending in 2 among the 100s are 102, 112, ..., 192: 10 numbers (those ending in 2 among the 200s are already counted). Total = 100 + 10 = 110. Remember: count one group, then add only the new members of the other, to avoid double counting.
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: 100 + 10 = 110.
- (b) 111 over-counts by one.
- (c) 112 over-counts by two.
- (d) 110 is available as option (a), so 'none' is wrong.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2016, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.