A boat running downstream covers a distance of 20 km in 2 hours while for covering the same distance upstream,it takes 5 hours.What is the speed of the boat in still water ?	Choose the correct choice from the following options:
A) 6 B) 7 C) 8 D) 9 E) 1

Rationale: explanation : rate downstream = ( 20 / 2 ) kmph = 10 kmph ; rate upstream = ( 20 / 5 ) kmph = 4 kmph speed in still water = 1 / 2 ( 10 + 4 ) kmph = 7 kmph answer : b
Answer: B