2003 General Studies, all questions
UPSC Prelims 2003 · General Studies
Question 120: Basic Numeracy
Three bells toll at intervals of 9, 12 and 15 minutes respectively. All three begin to toll at 8 a.m. At what time will they first toll together again?
a8.45 a.m.
b10.30 a.m.
c11.00 a.m.
d1.30 p.m.
AnswerThe answer is (c).
The bells toll together again after the LCM of their intervals. 9 = 3 x 3, 12 = 2 x 2 x 3 and 15 = 3 x 5, so LCM = 2 x 2 x 3 x 3 x 5 = 180 minutes = 3 hours. Starting together at 8 a.m., they next toll together at 11.00 a.m.
Why each option is right or wrong
- (a) 8.45 is 45 minutes later, which is not a multiple of 12.
- (b) 10.30 is 150 minutes later, which is not a multiple of 9 or 12.
- (c) Correct: 180 minutes later is 11.00.
- (d) 1.30 p.m. is 330 minutes later, which is not a multiple of 9 or 12.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2003, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.