Question

An "A" train leaves a subway station every 12 minutes. An "E" train leaves every 9 minutes. If both trains just left the station on parallel tracks, when will both leave the station together again?

A. in 108 minutes
B. in 36 minutes
C. in 72 minutes
D. in 24 minutes

Answer 1

This problem is asking you to find the common multiple 

So if you make a list of all the multiples of each number (9*1 9*2 9*3)

The number that each has in common is 36

The answer is 36