Question

If one bell rings every 5 seconds and the other one rings every 10 seconds starting from 0 seconds, then they will ring together at _____ seconds.

• A. ${25}^{\text{th}}$
• B. ${35}^{\text{th}}$
• C. ${45}^{\text{th}}$
• D. ${20}^{\text{th}}$

Answer 1

The correct option is D ${20}^{\text{th}}$

Multiples of 5 = 5, 10, 15, 20, 25, 30, 35, 40 ...
Multiples of 10 = 10, 20, 30, 40, 50, 60, 70...

The bells will ring together at the seconds which are multiples of both 5 and 10.

So starting from 0, the two bells will ring together at 20th, 30th and 40th seconds etc.