A and B walk around a circular track. They start at 9 a.m. from the same point in the opposite directions. A and B walk at a speed of 2 rounds per hour and 3 rounds per hour respectively. How many times shall they cross each other before 11:00 a.m.?  Choose the most appropriate option
A) 8
B) 7
C) 6
D) 15
E) 3
Sol.   Relative speed = (2 + 3)= 
<gadget id="calculator">(2 + 3)</gadget>
<output>5</output>
5 rounds per hour.   So, they cross each other 5 times in an hour.   Hence, they cross each other 15 times before 11:00 a.m.   Answer 15
<result>D</result>
A and B walk around a circular track. They start at 9 a.m. from the same point in the opposite directions. A and B walk at a speed of 2 rounds per hour and 3 rounds per hour respectively. How many times shall they cross each other before 11:00 a.m.?