In the sequence 1, 5, 7, 3, 5, 7, 4, 3, 5, 7, how many such 5s are there which are not immediately preceded by 3 but are immediately followed by 7 ?
- (a) 1 ✓ UPSC's answer
- (b) 2
- (c) 3
- (d) None
Why the answer is (a)
• The sequence is: 1, 5, 7, 3, 5, 7, 4, 3, 5, 7.
• We need to find 5s that are immediately followed by 7.
• The 5s followed by 7 are at positions 2 (1, 5, 7), 5 (3, 5, 7), and 9 (3, 5, 7).
• Check the preceding number for each: Position 2 is preceded by 1 (not 3), so it qualifies.
• Position 5 is preceded by 3, so it does not qualify.
• Position 9 is preceded by 3, so it does not qualify.
• Only one 5 meets both conditions, so the answer is 1.
Why the other options are wrong
- (b) 2
- Only the 5 in position 2 qualifies; the other two 5s that are followed by 7 are preceded by 3.
- (c) 3
- Three 5s are followed by 7, but two of them are preceded by 3, so only one qualifies.
- (d) None
- The 5 in position 2 (preceded by 1, followed by 7) qualifies, so the answer is not 'None'.
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.