7/8 = x/24
8*x=7*24
8x=168
x=168:8
x=21
5/9 = 10/x
5*x=10*9
5x=90
x=90:5
=18
5/1,2 = x/6
1,2x=6*5
1,2x=30
x=30:1,2
x=25
3,5/4 = x/20
4x=20*3,5
4x=70
x=70:4
x=17,5
1,9/3 = x/15
3x=15*1,9
3x=28,5
x=28,5:3
x=9,5
4,1/7 = 20,5/y
4,1y=20,5*7
4,1y=143,5
y=143,5:4,1
y=35