For speeds between 40 and 65 miles per hour, a truck gets 480/x miles per gallon when

Question:

For speeds between 40 and 65 miles per hour, a truck gets 480/x miles per gallon when driven at a constant speed of x miles per hour. Diesel gasoline costs $3.90 per gallon, and the driver is paid $19.50 per hour. What is the most economical constant speed between 40 and 65 miles per hour at which to drive the truck?

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