23.5+6.5*4.2
=23.5+6.5+6.5×3.2
=30+6.5×（3+0.2）
=30+19.5+1.3
=49.5+1.3
=50.8
9.5*3*2.4
=（9.5×2）×（3×1.2）
=19×3.6
=（20-1）×3.6
=20×3.6-3.6
=72-3.6
=68.4
6.4*1.25
=0.8×（8×1.25）
=0.8×10
=8