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8 : 2 = х : 4
х : 5 = 15 : 3
81 : 27 = 21 : х
100 : х = 40 : 10

Пошаговое объяснение:

8 : 2 = х : 4

2*x = 8*4

2x = 32

x = 32 : 2

x = 16

х : 5 = 15 : 3

3*x = 15*5

3x = 75

x = 75:3

x = 25

81 : 27 = 21 : х

81*x = 27*21

81x = 567

x = 567:81

x = 7

100 : х = 40 : 10

40*x = 100*10

40x = 1000

x = 1000 : 40

x = 25

8/2=х/4

x=8*4/2

x=16

x/5=15/3

x=5*15/3

x=25

81/27=21/x

x=27*21/81

x=7

100/x=40/10

x=100*10/40

x=25