Rohit and Sameer start together from the same point of a circular track 600 m round and run in opposite directions at 4 m/s and 6 m/s. They meet for the first time after how many seconds?

Running in opposite directions their speeds add, so between them they cover 4 + 6 = 10 m every second, and a first meeting needs one full lap of 600 m to be covered. Time = 600/10 = 60 s. (300 s is the answer for the same direction, where the relative speed is only 6 − 4 = 2 m/s.)