3.4x80%+0.8+80%x5.5
=3.4×0.8+0.8+0.8×5.5
=0.8×（3.4+1+5.5）
=0.8×9.9
=0.8×（10-0.1）
=0.8×10-0.8×0.1
=8-0.08
=7.92