UPSC Prelims 2019 · CSAT
Question 12: Logical Reasoning and Mental Ability
Mr 'X' has three children. The birthday of the first child falls on the 5th Monday of April, that of the second one falls on the 5th Thursday of November. On which day is the birthday of his third child, which falls on 20th December ?
AnswerUPSC’s answer is (b).
A 30-day month has a fifth Thursday only if its first day is a Wednesday or Thursday, so the fifth Thursday of November is the 29th or the 30th. If it were the 30th, 20 December would be 20 days later, a Wednesday, which is not an option. So it is the 29th, and 20 December is exactly 21 days after it, which is again a Thursday. The first child's date is not needed.
Why each option is right or wrong
- (a) Wrong: neither 29 nor 30 November being a Thursday leads to a Monday.
- (b) Right: 21 days after Thursday 29 November is a Thursday.
- (c) Wrong: Saturday is not reached from either possible fifth Thursday.
- (d) Wrong: Sunday is not reached from either possible fifth Thursday.
Worth knowing: In a single year a fifth Monday of April and a fifth Thursday of November cannot both occur, so the data conflict; the answer works only by elimination, taking 29 November as the fifth Thursday.
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.