＝（500-10）x0.35＝500x0.35-10x0.35＝175-3.5＝171.5
490X0.35=500×0.35-10×0.35=175-3.5=171.5
490X0.35的简便算法=49x3.5=50x3.5-3.5=175-3.5=171.5
