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 6 mph, how fast must anthony travel in order to return in 10 hours?

Let "x" be the speed of the boat.

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

4x+24=6x-36.

6x-4x=36+24.

2x=60.

x=60/2.

=30 mph.