Aboat in still water is 32 mph. if the boat travels 72 miles upstream in the same time that it takes to travel 120 miles downstream, find the speed of the current.

Well, keeping in mind, the boat's speed is still water is 32mph

now, when the boat goes upstream, is not really going 32mph, is going slower, because the river stream is eroding speed from it, let's say the stream rate is "r" fast, so the boat is going upstream at 32 - r

and when going downstream, it goes "with" the stream, so the boat is really going faster than 32mph, is going 32 + r

now, recall your d = rt, or distance = rate*time

keep in mind, the time, whatever that is, is the same for the boat in this case, same time going up as going down

thus  

solve for "r"