解原式=1.25x0.8x7x0.2=1x7x0.2=1.4
1.25×7×0.8×0.2＝1.25×0.8×7×0.2＝（1.25×0.8）×（7×0.2）＝1×1.4＝1.4
=（1.25×0.8)×7×0.2=1×7×0.2=1.4
