On a weekend outing, Anthony rents a motorboat for 10 hours to travel down the river and back. The rental operator tells him to go for 4 hours downstream, leaving 6 hours to return upstream. If the river current flows at a speed of 4 mph, how fast must anthony travel in order to return in 10 hours?

Let "x" be the speed of motor boat.

S/(x+4)=4.

S/(x-4)=6.

(x-4)/(x+4)=4/6.

(x-4)/(x+4)=2/3.

3(x-4)=2(x+4).

3x-12=2x+8.

x=20 mph.