Question

A traffic light at a crossing changes after every 40 seconds.The traffic light at the next crossing changes after every 30 seconds.At a certain time both the lights change together. After how many minutes of their changing simultaneously will they change again at the same time

Answer 1

the required answer is the L.C.M of 30 and 40
which is 120
Therefore,both the traffic ight would change after 1min20s
hope it helps