arithmetic - a motorcyclist x drives along a circular fence at a rate of 2 rounds per hour and another motor cyclist y at a rate of 4 rounds per hour after how long they will cross each other for the first time - skillgun

Since X and Y move in the same direction they cross each other when there is a difference of two round between the two. Relative speed of X and Y=4-2=2.
Time taken to complete one round at this speed = 1/2 hour
= 30minutes