Question

Two lighthouses can be seen from the top of the hill. The first flashes once every 3 seconds and the other flashes once every 5 seconds. If they flash at the same time, how long will it be before they flash at the same time again?​

Answer 1

We know that first flashes once every 3 seconds and the other flashes once every 5 seconds,

Therefore we need to take the LCM of those two numbers

3- 3,6,9,12,15

5- 5,10,15

Therefore after 15 seconds they both will flash at the same time.