Three athletes X, Y and Z start running around a circular track of length 60m from the same point simultaneously in the same direction at speeds of 3 m/s, 5 m/s, and 9 m/s respectively. After how long will they meet for the first time after they started running?

Correct Answer: 60 sec.
Time take by X to cover a circle = 60/3 = 20 sec Time taken by Y to cover a circle = 60/5 = 12 sec Time taken by Z to cover a circle = 60/9 sec   So every 20 sec, X will be at the starting point. Every 12 secs Y will be at the starting point. Every 60/9 sec, Z will be at the starting point. Taking their LCM, we get 60. So every 60 sec, all three will be at the starting point.  All meet for the first time at the starting point after they start moving after 60 sec.