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?
Choices:
A) 10 min B) 20 min C) 30 min D) 40 min E) None of these
Explanation :   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= 
<gadget id="calculator">4-2</gadget>
<output>2</output>
2.   Time taken to complete one round at this speed= 1/2 hour   = 30minutes   Answer : 30 min
<result>C</result>
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?