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1 )

x + 4/21 = 2/3 + 2/7

x + 4/21 = 14/21 + 6/21

x + 4/21 = 20/21

x = 20/21 - 4/21

x = 16/21

3 ) (4/5 - x) + 13/20 = 7/10

4/5 - x = 7/10 - 13/20

4/5 - x = 1/20

-x = 1/20 - 4/5

-x = 1/20 - 16/20

-x =  - 15/20

x = 15/20

x = 5/4

2)

y  + 1/4 = 4/5 - 3/10

y = 4/5 - 1/4 - 3/10

y = 16/20 - 5 /20 - 6/20

y = 5/20

y = 1/5

4)

2/3 - 2/9 -x = 1/3

6/9 - 2/9 - x = 1/3

-x = 1/3 - 6/9 + 2/9

-x = 3/9 - 6/9 + 2/9

-x = - 1/9

x = 1/9