Question

Traffic lights at three different crossings on a road change after every 10
15 minutes and 20 minutes, respectively. If they change
together at 9 a.m. first, when will they change together
again?​

Answer 1

We'll take LCM of all the three at that point they will change together again.

LCM ( of 10, 15 , 20 ) = 30

Hence after 30min they will change together again.

That is at 9:30 am

Hope the solution is clear to you!!