The speed of a boat in still water is 36 kmph. What is the speed of the stream if the boat can cover 80 km downstream or 40 km upstream in the same time?  Pick
A) 10 kmph
B) 14 kmph
C) 12 kmph
D) 16 kmph
E) 15 kmph

Rationale: "x = the speed of the stream ( 36 + x ) / ( 36 - x ) = 2 / 1 36 + x = 72 - 2 x 3 x = 36 x = 12 km / hour if the speed of the stream is 12 km / hour , then the ' downstream ' speed of the boat is 36 + 12 = 48 km / hour and the ' upstream ' speed of the boat is 36 - 12 = 24 km / hour . in that way , if the boat traveled for 2 hours , it would travel 2 x 48 = 96 km downstream and 2 x 24 = 48 km / hour upstream . answer : c"
Answer: C