数学题，50分！！！！！！！！过程

0.25÷1又1/2+0.75*2/3
= 1/4 *2/3 +3/4 *2/3
= (1/4+3/4)*2/3
= 1*2/3
= 2/3

7又3/4*2又1/4÷5又1/6+4又5/6
= 31/4 *9/4 *6/31 +4又5/6
= 27/8 +4又5/6
= 3又3/8 +4又5/6
= (3+4)+(3/8+5/6)
= 7 +(9/24+20/24)
= 7 +1又5/24
= 8又5/24

2又2/9*1.2÷2/3
= 20/9 *6/5 *3/2
= 4

[2.1+7÷（3又1/2-1又5/8）]*1又2/3
=[2.1+7÷1又7/8]*1又2/3
=[2.1+7*8/15]*1又2/3
=[21/10+56/15]*5/3
=21/10*5/3+56/15*5/3
=3又1/2+6又2/9
=(3+6)+(1/2+2/9)
=9+13/18
=9又13/18