Four runners started running simultaneously from a point on a circular track. They took 400 seconds, 600 seconds, 720 seconds and 900 seconds to complete one round. After how much time did they meet at the starting point for the first time since the race started?

3600 sec

During their individual time , all of them cover different number of round .

So , meeting time will be LCM of individual's time

LCM of ( 400 , 600 , 720 , 900 ) = 3600 sec

So , they will meet after 3600 sec