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Four bells begin tolling at the same time and continue to toll at intervals of 20, 25, 28 and 50 seconds respectively. Find the smallest integral numbers of minutes after which all the bells toll together again.

A.	700	B.	70
C.	1400	D.	35

Answer and Explanation

Answer:35

Explanation:
The smallest time after which all the bells toll together again = K × LCM of (20, 25, 28, 50), where K = 1, 2, 3…
20 = 2 × 10 = 22 × 5; 25 = 52
28 = 2 × 2 × 7 = 22 × 7; 50 = 5 × 10 = 2 × 52
Time = K × LCM of (20, 25, 28, 50) = K × 22 × 52 × 7 = 700k sec
When K – 1, Time = 700 sec, which is not an integral minute.
When K = 3, Time = 3 × 700 sec = 2100 sec = 35 minutes.

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