Question

A northbound bus returns to the bus stop every 20 minutes. The southbound bus returns to the bus stop every 25 minutes.How long will it be before both of the buses are at the bus stop at the same time again

Answer 1

They  stop at the  same time after 4  times for  the southbound bus to return and five times for the northbound bus returns to the bus stop
 

Answer 2

100 minutes because
20,40,60,80,100,120
25,50,75,100,125
They have 100 in common. It will take the northbound bus 5 stops and the southbound bus 4 stops to be at the same bus stop