A small boat traveling downstream with the current covers a distance of 3 miles in 1/3 hour. The return trip, against the current, takes 1/2 hour. Assume that the speed of the current is the same during both trips. What is the speed of the boat in still water?

call unknowns variables; let us call speed of boat in still water x and current speed y.

since net speed is 3 miles per 1/3 hour (by first fact) or 9mph, we can say x+y=9.

On the return trip, the net speed is 3 miles per 1/2 hours (by second fact) or 6 mph, so we can say x-y=6.

Two variables, two equations, it is linear...solve