3/4:12/11=x:20/1
12/11x=3/4×20/1
12/11x=15/1
x=55/11
x:4.5=5.8:2.9
x:4.5=2:1
x=4.5×2
x=9
1又6/1:5/2=3又3/1:x
6/7:5/2=3/10:x
6/7x=3/10×5/2
6/7x=3/4
x=3/4×7/6
x=7/8
10:x=4又2/1:0.8
10:x=4.5:0.8
4.5x=10×0.8
4.5x=8
x=9/16