解方程：（6.8-X)*9/11=1.21x-3/20x=423x*2/3=8/9用简便方法计算：16/21除以4+3/

解方程：（6.8-X)*9/11=1.21
6.8-X=1.21*11/9
x=6.8-1.21*11/9
=6.8-13.31/9
=4789/900
=5又289/900
x-3/20x=42
17/20x=42
x=840/17
3x*2/3=8/9
2x=8/9
x=4/9
用简便方法计算：16/21除以4+3/4*16/21
=4/21+12/21
=16/21
5/42+6.85+1.6+43/20
=5/42+6又17/20+1.6+43/20
=5/42+9+1.6
=5/42+10+0.6
=5/42+10+3/5
=10+25/210+126/210
=10+134/210
=10又134/210
(108+45+72/73)除以9
=108/9+45/9+72/（73*9）
=12+5+8/73
=17又8/73
51/4除以6/5+5.25*5/6
=51/4*5/6+5.25*5/6
=（51/4+5.25）*5/6
=（12.75+5.25）*5/6
=18*5/6
=15