7.5×23+31×2.5 0.79×0.46+7.9×0.24+11.4×0.079
=2.5×3×23+31×2.5 =7.9×0.046+7.9×0.24+7.9×0.114（根据积不变的规律）
=2.5×（69+31） =7.9×（0.046+0.24+0.114）（根据乘法分配律）
=2.5×100 =7.9×0.4
=250 =3.16