Question 256224
Let the speed of the boat in still water = x
And the speed of the stream              = y
When the boat goes downstream, the resultant speed = x+y
Given,A motorboat can go 16 miles downstream on a river in 20 minutes. 
1) So we have, distance = speed*time
                     16 = (x+y)*(20/60)
                     16 = (x+y)*(1/3)
                     48 = x+y
                     x+y=48    ...(1)
2) When the boat goes upstream, the resultant speed = x-y
It takes 30 minutes for this boat to go back upstream the same 16 miles.
So we have, distance = speed*time
                     16 = (x-y)*(30/60)
                     16 = (x-y)*(1/2)
                     32 = x-y
                     x-y=32    ...(2)
3) Solving (1) and (2),
(1)+(2)=>            2x = 48+32
                     2x = 80
                      x = 80/2
                      x = 40
Substituting in (1), we get
                    40+y = 48
                       y = 48-40
                       y = 8