解；
3/8:9/10=x:2/5
∴
9/10x=3/8*2/5
9/10x=3/20
x=3/20*10÷9
x=3/2*1/9
x=1/6
1/2:2/7=1/3:4x
∴
2x=2/7*1/3
2x=2/21
x=1/21
0.5/x=5,2/4
∴
5.2x=4*0.5
x=2÷5.2
x=2*26/5
x=52/5
(x-1)/(3/5)=0.1/(3/5
∴x-1=0.1
x=1.1