A boat crossed a lake from North to East at the speed of 10 km/h, entered a river and covered twice as much distance going upstream at 9 km/h. It then turned around and stopped at the south shore of the lake. If it averaged 3.8 km/h that day, what was its approximate downstream speed?	Choose the correct choice from the following choices.
A) 4  B) 5  C) 6  D) 7  E) 11

Rationale: one way of solving this is : speed of boat on still water ( lake ) = 10 kmph speed upstream = 9 kmph = speed in still water - speed of river = > speed of river = 1 kmph = > speed downstream = speed in still water + speed of river = 10 + 1 = 11 kmph ans is e
Answer: E