d=vt (distance is equal to velocity times time)
let d=distance, b=velocity of boat, s=velocity of stream, and t=time
since d=vt, t=d/v
So for each trip we have:
t=11/(b-s) which is t=11/(b-3)
And for the downstream trip we have:
t=17/(b+s) which is t=17/(b+3)
Since we are told that each trip takes the same amount of time, t=t so we can say:
17/(b+3)=11/(b-3) cross multiply we have:
17(b-3)=11(b+3)
17b-51=11b+33
6b=84
b=14
So the speed of the boat in still water is 14mph.
