Three alarm clocks ring at intervals of 6,9 and 15 minutes respectively. If they start ringing together, after what time will they next ring together?

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120 minutes

90 minutes

110 minutes

60 minutes

answer is B.

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Detailed Solution

Given,
Three alarm clocks ring at intervals of 6, 9 and 15 minutes respectively.
The LCM of 6, 9 and 15 are,

⇒ 6 = 2 × 3

⇒ 9 = 32

⇒ 15 = 3 × 5

⇒ L C M (6, 9, 15) = 2 × 32 × 5 ⇒ L C M = 90

Thus, after 90 minutes all three alarms will ring together.
Therefore, option(2) is correct answer.

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