Question

Jack went to a church. He heard the sound of two bells ringing at the same time.
He heard the bells ringing individually after intervals of 15 minutes and 20 minutes.
Find out at what interval Jack will hear the bells ringing together again.

• A. 90 minutes
• B. 60 minutes
• C. 45 minutes
• D. 80 minutes

Answer 1

The correct option is B 60 minutes

Let A be the bell that rings at intervals of 15 minutes and B be the bell that rings at intervals of 20 minutes.

To find after how many minutes both bells will ring together, we have to find the LCM:

Multiples of 15: 15, 30, 45, 60, 75, 90, ...
Multiples of 20: 20, 40, 60, 80, 100, 120, ...

The least common multiple of 15 and 20 is 60.

Thus, Jack will hear the bells ringing together again after 60 minutes.