A boat covers a certain distance downstream in 1 hour, while it comes back in 1½ hours. If the speed of the stream be 3 kmph, what is the speed of the boat in still water?
Choose the correct option:  A) 12 kmph  B) 13 kmph  C) 14 kmph  D) 15 kmph  E) None
Sol.   Let the speed of the boat in still water be x kmph. Then,   Speed downstream = (x + 3) kmph, Speed upstream = (x - 3)kmph.   ∴ (x + 3) * 1 = (x -3) * 3/2 ⇔ 2x + 6 = 3x - 9 ⇔ x = 15 kmph.   Answer 15 kmph
<result>D</result>
A boat covers a certain distance downstream in 1 hour, while it comes back in 1½ hours. If the speed of the stream be 3 kmph, what is the speed of the boat in still water?