Question

on a street corner there are two flashing lights, a red one and a blue one. The red one flashes three times per minute, and the blue one flashes two times per minute. if the lights start off flashing at the same time, once every how many seconds do they flash together?

Answer 1

The red one flashes once per 20 secs; the blue one once per 30 secs.
They will flash together every:
Least common multiple (20;30) = 60 seconds.

Answer 2

Why is it least common multiple?