5.6×3.5=0.7×8×0.7×5=(0.7×0.7)×（8×5）=0.49×40=4.9×4=19.6 5.6×3.5=7×0.8×0.7×5=（7×0.7）×（0.8×5）=4.9×4=（5-0.1）×4=5×4-0.1×4=20-0.4=19.6
