A truck gets 600/x miles per gallon (mpg) when driven at a constant speed of x mph, where 40 x 80. If the price of fuel is $2.

A truck gets 600/x miles per gallon (mpg) when driven at a constant speed of x mph, where 40 ≤ x ≤ 80. If the price of fuel is $2.80/gal and the driver is paid $13/hr, at what speed is it most economical for the trucker to drive?