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
=======================
16/21 + 4/21 = 2/3 + 2/7
20/21 = 14/21 + 6/21
20/21 = 20/21
y + 1/4 = 4/5 - 3/10
y + 1/4 = 8/10 - 3/10
y + 1/4 = 5/10
y + 1/4 = 1/2
y = 1/2 - 1/4 = 2/4 - 1/4
y = 1/4
==========================
1/4 + 1/4 =4/5 - 3/10
2/4 = 8/10 - 3/10
1/2 = 5/10
1/2 = 1/2
(4/5 - x) + 13/20 = 7/10
4/5 - x = 7/10 - 13/20
4/5 - x = 14/20 - 13/20
4/5 - x = 1/20
x= 4/5 - 1/20 = 16/20 - 1/20 = 15/20
x= 3/4
=============================
(4/5 - 3/4) + 13/20 = 7/10
(16/20 - 15/20) + 13/20 = 7/10
1/20 + 13/20 = 7/10
14/20 = 7/10
7/10 = 7/10
2/3 - (2/9 + x) = 1/3
2/3 - 2/9 - x = 1/3
6/9 - 2/9 - x = 1/3
4/9 - x = 3/9
x= 4/9 - 3/9
x= 1/9
========================
2/3 - (2/9 + 1/9) = 1/3
2/3 - 3/9 = 1/3
2/3 - 1/3 = 1/3
1/3 = 1/3