Its called gauge pressure
If it takes 42 minutes to load 3 1/2 trucks, then it takes 42 / 3.5 = 12 minutes to load a single truck. Multiplying the number of minutes per truck by 6.5 yields the time it will take to load. In this case 12 x 6.5 = 78 minutes would be required to load 6 1/2 trucks. The formula for the time taken is t = 12n, where t is the time in minutes and n is the number of trucks.
