x：6.5=6:4
x=6.5*6/4
x=9.75
5:8=x：16
x=5*16/8
x=10
0.3/1.5=4.2/x
x=1.5*4.2/0.3
x=21
5/3:x=2:3/2
x=3/5*2/3/2
x=1/5
2/1:4=10/1:x
x=4*1/10/1/2
x=4/5
7/2=x/8
x=7*8/2
x=28