数学题，50分啊

先把50分给了。

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÷（7/2-13/8）]*5/3
=[2.1+7÷（28/8-13/8）]*5/3
=[2.1+7÷15/8]*5/3
=[21/10 +7*8/15]*5/3
= 21/10*5/3 +7*8/15*5/3
= 7/2 +56/9
= (3+1/2) +(6+2/9)
=(3+6)+(1/2+2/9)
= 9+(9/18+4/18)
= 9+13/18
= 9又13/18.