Ответ:

Пошаговое объяснение:

1)

х + 4/21 = 2/3 + 2/7

х + 4/21 = 14/21/ + 6/21

х + 4/21 = 20/21

х = 20/21 - 4/21

х = 16/21

2)

у + 1/4 = 4/5 - 3/10

у + 5/20 = 16/20 - 6/20

у + 5/20 = 10/20

у = 10/20 - 5/20

у = 5/20

у = 1/4

3)

(4/5 - х) + 13/20 = 7/10

4/5 - х = 7/10 - 13/20

16/20 - х = 14/20 - 13/20

16/20 - х = 1/20

х = 16/20 - 1/20

х = 15/20

4)

2/3 - (2/9 + х) = 1/3

2/9 + х = 2/3 - 1/3

2/9 + х = 1/3

х = 1/3 - 2/9

х = 3/9 - 2/9

х = 1/9