2.8×0.25×0.9＝4×0.25×0.7×0.9＝1×0.7×0.9＝0.63
2.8ⅹ0.25x0.9=0.7ⅹ4ⅹ0.25ⅹ0.9=0.7x1ⅹ0.9=0.63
2.8Ⅹ0.25X0.9=4X0.25X0.7X0.9=1X0.63=0.63
从2.8入手2.8＝4×0.7带入0.7×4×0.25×0.90.7×1×0.90.63
