5.125-（3又2分之1 +8分之3）
=5+0.125-(3+1/2+3/8)
=5+1/8-3-1/2-3/8
=5-3-2/4-3/8+1/8
=2-2/4-1/4
=2-3/4
=1又1/4
8分之3乘5又3分之1除以1.2-2分之1
=3/8*16/3*10/12-1/2
=5/3-1/2
=10/6-3/6
=7/6
=1又1/6
3分之8乘9乘12分之7
=8/3*9*7/12
=8*3*7/12
=8*7/6
=56/6
=9又1/3
1.2除以3又4分之3乘8分之3
=12/10*4/15*3/8
=12/10*1/5*1/2
=3/25
（7分之4+3分之2）乘8分之21
=4/7*21/8+2/3*21/8
=3/2+7/4
=6/4+7/4
=13/4
=3又1/4