John can row at 5 kmph and still water. He takes 5 hours to row from A to B and back. What is the distance between A and B if the speed of the stream is 1 kmph?
Choose the most appropriate option
A) 28 km B) 12 km C) 16 km D) 30 km E) 20 km

Rationale: let the distance between a and b be x km . total time = x / ( 5 + 1 ) + x / ( 5 - 1 ) = 5 = > x / 6 + x / 4 = 4 = > ( 2 x + 3 x ) / 12 = 5 = > x = 12 km . answer : b
Answer: B