man can row upstream at 30 kmph and downstream at 40 kmph, and then find the speed of the man in still water?  Choose the correct choice from the following answers
A) 30  B) 35  C) 40  D) 70  E) 45

Rationale: "us = 30 ds = 40 m = ( 30 + 40 ) / 2 = 35 answer : b"
Answer: B