Question

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

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

Answer 1

The correct option is C

3 hours

Tolling together for next time means tolling after the least possible minutes which is the LCM of 9, 12 and 15.

LCM = 2 $\times$ 2 $\times$ 3 $\times$ 3 $\times$ 1 $\times$1 $\times$ 5 = 180

$\therefore$ Time after which the three bells will toll together next = 180 minutes

We know that 60 minutes = 1 hour

$\therefore$ 180 minutes = $\frac{180}{60}$ = 3 hours