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

Given that the three bells toll at intervals – 9 min, 12 min, and 15min.

So, if they start tolling together, they will ring together at L.C.M (9,12,15)

To find the L.C.M of 9, 12 and 15

Prime factors of 9 = 3 × 3

Prime factors of 12 = 2 × 2 × 3

And Prime factors of 15 = 3 × 5

L.C.M (9, 12, 15) = 3 × 3 × 2 × 2 × 5 = 180

So, the L.C.M of 9, 12 and 15 is 180.

Hence, the bells will toll together after 180 minutes or 3 hours.