Question

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

• A. 3 hours
• B. 2 hours
• C. 1 hour
• D. 4 hours

Answer 1

The correct option is A 3 hours

Tolling together for next time means tolling after the least possible minute which is the L.C.M of 9, 12 and 15.

$\begin{array}{cc}2& 9,12,15\\ 3& 9,6,15\\ & 3,2,5\end{array}$

L.C.M of 9, 12 and 15

$=2\times 3\times 3\times 2\times 5$

$=180$ minutes

$=\frac{180}{60}$ hours

$=3$ hours

Hence, the bells will toll together after 3 hours.