green line buses run every 10 minutes
red line buses run every 20 minutes
purple line buses run every 35 minutes
one bus from each line leaves the city center at 9:00
after how many minutes will buses from all three lines next leave the city center at the same time?
you are looking for the LCM of 10, 20 and 35
10 = 2*5
20 = 2*2*5
35 = 5*7
LCM = 2*2*5*7 = 140
so it would be 140 minutes from 9:00 which would be .....