A motorboat can keep a constant speed of 16 mph relative to the water. The boat makea a trip upstream to a certain point in 20 minutes; the return trip takes 15 minutes. What is the speed of the current?

Let c = speed of current

I know that 20 minutes is 20/60 of an hour or 1/3 of an hour.

I also know that 15 minutes is 15/60 of an hour or 1/4 of an hour.

The formula D = rt is often used in such questions. I also think the formula must somehow be manipulated to show what we are looking for.

D = rt

Upstream = c + 16

Downstream = c - 16

I stuck here.