Question

Samaira heard 3 bells toll together at 10:00 AM. The first bell tolls after every 5 second, the second tolls after every 9 seconds and the third tolls after every 10 seconds. When will they toll together again?

• A. After 2 minutes and 30 seconds
• B. After 45 seconds
• C. After 3 minutes
• D. After 90 seconds

Answer 1

The correct option is D After 90 seconds

The bells will ring together at common times. And this time will be a multiple of all the times at which the 3 bells ring individually, that is 5, 9 and 10. We have to find the LCM (least common multiple) of these numbers to know after what time all the 3 bells toll together.

$\therefore \text{LCM of 5, 9, 10}=5\times 2\times 3\times 3$
= 90

So the bells will together after every 90 seconds.