Seeta and Geeta go for a swim after a gap of every 2 days and every 3 days respectively. If on 1ˢᵗ January both of them went for a swim together, when will they go together next ?
- (a) 7ᵗʰ January
- (b) 8ᵗʰ January
- (c) 12ᵗʰ January
- (d) 13ᵗʰ January ✓ UPSC's answer
Why the answer is (d)
• 'After a gap of every 2 days' means two rest days between swims, so Seeta swims every 3rd day: 1, 4, 7, 10, 13 January.
• 'After a gap of every 3 days' means three rest days between swims, so Geeta swims every 4th day: 1, 5, 9, 13 January.
• The LCM of 3 and 4 is 12, so they next swim together 12 days after 1 January, on 13 January, option (d).
Why the other options are wrong
- (a) 7ᵗʰ January
- On 7 January only Seeta swims; Geeta swims on 5 and 9 January.
- (b) 8ᵗʰ January
- Neither of them swims on 8 January.
- (c) 12ᵗʰ January
- Neither of them swims on 12 January; their next common day is 13 January.
Asked in the CSAT Paper II of the UPSC Civil Services Preliminary Examination 2019, held on 2 June 2019. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.