The speed of a stream is 2 mph. A boat travels 8 miles upstream in the same time it takes to travel 12 miles downstream. What is the speed of the boat in still water? The speed is ___ mph.

we solve as a proportion: 

downstream
(v+2)t=12 
t=12/(v+2) 

upstream:
by same reasoning 
t=8/(v-2) 

setting t's equal and simplifying 

v=10 mph 


:)