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The bells begin ringing at the same time and continued to do so at intervals of 21, 28, 30 seconds respectively. The bells will ring together again after

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7 seconds

420 seconds

630 seconds

1764 seconds

answer is B.

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Detailed Solution

The time after which they will toll together again must be a multiple of 21, 28 and 30. Hence LCM of 21, 28 and 30 is 420.

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