Question

A red light flashes every 10 seconds.

A green light flashes every 15 seconds.
They both just flashed.
How long before they flash together again?
A. 5 seconds
B. 15 seconds
C. 30 seconds
D. 150 seconds

Answer 1

C

Explanation:

find the LCM of 10 and 15

multiples of 10 are 10, 20, 30, 40, .....

multiples of 15 are 15, 30, 45, 60, ....

the LCM of 10 and 15 is 30

they will flash together again after 30 seconds