简便计算的方法如下：
6.4+3.6×0.42
=6.4+（3.2+0.4）×0.42
=6.4+3.2×0.42+0.4×0.42
=6.4+6.4×0.21+0.4×0.42
=6.4×（1+0.21）+0.4×0.42
=6.4×1.21+0.4×0.42
=0.8×0.8×1.21+0.8×0.21
=0.8×（0.8×1.21+0.21）
=0.8×（0.968+0.21）
=0.8×1.178
=0.8×（1.2-0.02-0.002）
=0.96-0.016-0.0016
=0.944-0.0016
=0.9424
