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 10.30 a.m.?  Choose the correct choice from the following answers
A) 8
B) 7
C) 12
D) 5
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 and 2 times in half an hour.   Hence, they cross each other 12 times before 10.30 a.m.   Answer 12
<result>C</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 10.30 a.m.?