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?  Select the correct option
A) 100 km  B) 150 km  C) 80 km  D) 120 km  E) 200 km

Rationale: "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 c"
Answer: C