The speed of a stream is 5 mph. If a boat travels 72 miles downstream in the same time that it takes to travel 36 miles upstream, what is the speed of the boat in still water?

----

Downstream DATA:

distance = 72 miles ; rate = b+5 mph ; time = 72/(b+5) hrs.

----

Upstream DATA:

distance = 36 miles ; rate = b-5 mph ; time = 36/(b-5) hrs

----

Equation:

time = time

72(b+5) = 36/(b-5)

-------

2/(b+5) = 1/(b-5)

Cross-multiply to get:

2b-10 = b+5

b = 15 mph (speed of the boat in still water)