7.2加(4又3分之2减3又2分之1)乘6减(0.64乘25减2.8)
=7.2+7/6x6-16+2.8
=7.2+2.8+7-16
=10+7-16
=1
21又19分之1乘20分之1
=(20+20/19)x1/20
=20x1/20+20/19x1/20
=1+1/19
=1又1/19