For a construction job, a contractor can haul rock by means of an ordinary 10 cubic yard dump truck, or he can use a combination rig where a 10 cubic yard also pulls an 8 cubic yard trailer. When this combination is used, the trailer slides into the empty dump truck body for the return trip. The total equivalent operating cost, including the driver, amortization of capital, interest on the investment, fuel, and maintenance is 625/hour for the dump truck and 875/hour for the tandem rig. For the required 10-mile haul, the loading and unloading, hauling, and return times are as follows;

Time, minutes. 

Loading 3,5 

Unloading 4,10 

Hauling 24,28 

Return 20,24. 

How much is the difference in cost per unit volume between the two alternatives?r