Question 24642
Let speed of the boat in still water be "b".
Upstream Data:
   rate = b - 3 ;  time = 5 hrs.
Downstream Data:
   rate = b + 3 ;  time = 5/2 hrs.
EQUATION Data:
  Distance up = distance down and d=rt
  (b-3)5 = (b+3)(5/2)
Multiply thru by "2" to get:
  10(b-3) = 5(b+3)
  10b-30 = 5b+15
   5b = 45
    b = 9 mph (speed of boat in still water)

Cheers,
stan H