A man can row upstream at 25 kmph and downstream at 31 kmph, and then find the speed of the man in still water?  Pick:
A) 22
B) 28
C) 30
D) 27
E) 18

Rationale: "us = 25 ds = 31 m = ( 31 + 25 ) / 2 = 28 answer : b"
Answer: B