A motorboat can go at 21 kmph and still water. It takes 7 hours to row from A to B and back. What is the distance between A and B if the speed of the stream is 3 kmph? Choose the correct choice from the following: A) 65 km B) 85 km C) 55 km D) 72 km E) 40 km

Rationale: let the distance between a and b be x km . total time = x / ( 21 + 3 ) + x / ( 21 - 3 ) = 7 = > x / 24 + x / 18 = 7 = > ( 4 x + 3 x ) / 72 = 7 = > x = 72 km . answer : d
Answer: D