挖掘机台数×挖掘机平均挖土量=装卸车辆数×装卸车平均运土量设挖掘机的台数为x.750x = 300 × (21 - x)750x = 6300 - 300x750x + 300x = 63001050x = 6300x = 6300 ÷ 1050x = 621 - 6 = 15辆答：挖掘机有6台,装卸...