Posted by Jacob on .
A truck gets
800/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 $19/hr, at what speed is it most economical for the trucker to drive? (Round your answer to two decimal places.)
I have no idea how I should set this problem up.

Applied Calculus 
Reiny,
bobpursley and Steve gave solutions to very similar questions, you simply have to change the numbers
http://www.jiskha.com/display.cgi?id=1320423725
http://www.jiskha.com/display.cgi?id=1320423629