Truck mileage. The equation ? = 45.683-0.005211x gives the approximate gas mileage (in miles per gallon) for a light truck that weighs x pounds. The equation is valid for 4000 ? x ?6000.a) How many miles per gallon does the equation predict a light truck weighing 4750 pounds will get? Round to the nearest tenth.b) Why can't the equation be used to find the mileage for a truck weighing 6500 pounds?