Question

a bell rings every 6 seconds. a horn sounds every 10. when will they next sound together?

Answer 1

In 30 seconds, then in sixty.

Bell rings in  6, 12, 18, 24, 30, 36, 42, 48, 54 and 60 seconds

Horn rings in 10, 20, 30, 40, 50 and 60 seconds.