420-х=400:5
420-х=80
420-х-80=0
х=360
3х - 3 + х = 2х
4х - 2х = 3
2х = 3
х = 1,5
----------------------------
5х + 20 + х = 6
6х = -20 + 6
6х = - 14
х = - 2 1/3
------------------------------
3х - 21- 6х = - х
-3х + х = 21
2х = - 21
х = - 10,5
-----------------------------
7 - 2х - 3 = 9
- 2х = 9 - 7 + 3
2х = - 5
х = - 2,5
----------------------------
4х + 3х - 21 = 5
7х = 21 + 5
7х = 26
х = 3 5/7
6 + 1 = 7
6 + 2 = 8
6 + 3= 9
6 + 4 = 10
6 + 5 =11
6 + 6 = 12
6 + 7 = 13
6 + 8 = 14
6 + 9 = 15