Question 4056
 Let the speed of the current be x mph.
 [Note x should be less  than 15 mph, for otherwise theboat cannot move
 when gotupstream.]

 Downstream speed = x + 15. (sum of the speeds)
 Upstream speed = 15-x, (difference of the speeds)

 Now we have 12/(x+15) = 9/(15-x) or
 4(15-x) = 3(x+15)
 so 7x = 15 , x = 15/7 mph.

 You did completely wrong , try to think about it.

 Kenny