Earth whose in situ weight is 105lb/cf and whose compacted weight is 122 lb/cf is placed in a fill at the rate of 260 cy/hr, measured as compacted earth. The thickness of the compacted layers is 7 in. A roller having a drum width of 50 in operates at a speed of 2 mph. Assume a 55-min hour operation efficiency. Determine the number of rollers required to provide the necessary compaction if four passes are specified for each layer of earth.