75:(2х)=15:2544:99=4:(3х)(5х):13=8:5210,5:21=(4х):20

2х *15  = 75 * 25
30х = 1875
    х = 62,5
--------------------------
3х * 44 = 99 * 4
132х = 396
      х = 3
-------------------------
5х * 52 = 13 * 8
260х = 104
      х = 0,4
-------------------------
4х * 21 = 10,5 * 20
84х = 210
    х = 2,5

1)
75 : 2х = 15/25
75 : 2х = 3/5
2х = 75 3/5
2х = 45
х = 22,5

2)
44:99 = 4 : 3х
44/99 = 4/3х
4/9 = 4/3х
43х = 9 4
12x = 36
x = 3

3)
5x : 13 = 8:52
5x/13 = 8/52
5x/13 = 2/13
5x 13 = 13 2
65x = 26
x = 0,4

4)
10,5 : 21 = 4x : 20
10,5/21 = 4x/20
10,5 20 = 21 4x
210 = 84x
x = 2,5