In a row 'A' is in the 11th position from the left and 'B' is in the 10th position from the right. If 'A' and 'B' interchange, then 'A' becomes 18th from the left. How many persons are there in the row other than 'A' and 'B' ?
- (a) 27
- (b) 26
- (c) 25 ✓ UPSC's answer
- (d) 24
Why the answer is (c)
• Initially, A is at the 11th position from the left, so A's position is 11.
• B is at the 10th position from the right. Let the total number of persons be N. Then B's position from the left is N - 10 + 1 = N - 9.
• When A and B interchange positions, A moves to B's original position.
• After interchanging, A is 18th from the left. This means B's original position from the left was 18.
• Therefore, N - 9 = 18, which implies N = 27.
• The total number of persons in the row is 27.
• The question asks for the number of persons other than A and B, which is 27 - 2 = 25.
Why the other options are wrong
- (a) 27
- 27 is the total number of persons in the row, not the count excluding A and B.
- (b) 26
- 26 is incorrect because the total number of persons is 27, so excluding two leaves 25.
- (d) 24
- 24 is incorrect because the total number of persons is 27, so excluding two leaves 25.
Asked in the CSAT Paper II of the UPSC Civil Services Preliminary Examination 2014, held on 24 August 2014. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.