Three bells toll together at intervals of 9, 12 and 15 minutes. If they start tolling together, after what time will they toll next?

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$2 \frac{1}{2}$ hour

1 hour

$1 \frac{1}{2}$ hour

3 hours

answer is D.

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Detailed Solution

$\begin{array}{l} 3 9, 12, 15 \\ 3, 4, 5 \end{array}$
$\begin{array}{l} L . C . M = 3 \times 3 \times 4 \times 5 = 180 minutes \\ = \frac{180}{60} = 3 hours . \end{array}$

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