Question

Two lighthouses can be seen from the top of a hill.The frist flashes once every 6 seconds and the otherflashes once every 15 seconds.

If they flash simultaneously, how long is it until they flash again at the same time?

Answer 1

30 seconds

Explanation:

the second lighthouse flashes every 15 seconds. for the first 15, the other house will not flash because 15/6 is not a whole number. in the next 15 seconds, which is 30 seconds in all, the other will flash because 30/6 is a whole number.