A man can swim in still water at 15 km/h, but takes twice as long to swim upstream than downstream. The speed of the stream is? Choose the correct choice from the following answers:  A) 3.9  B) 5  C) 5.3  D) 1.5  E) 5.2

Rationale: "m = 15 s = x ds = 15 + x us = 15 - x 15 + x = ( 15 - x ) 2 15 + x = 30 - 2 x 3 x = 15 x = 5 answer : b"
Answer: B