A walks at 10 kmph and 4 hours after his start, B cycles after him at 20 kmph. How far from the start does B catch up with A?	Choose the correct answer.
A) 100km  B) 150km  C) 80km  D) 120km  E) 200km
Suppose after x km from the start B catches up with A. Then, the difference in the time taken by A to cover x km and that taken by B to cover x km is 4 hours.   x/10 - x/20 = 4   x = 80 km   Answer is 80km
<result>C</result>
A walks at 10 kmph and 4 hours after his start, B cycles after him at 20 kmph. How far from the start does B catch up with A?