Пошаговое объяснение:
1) x + 20 = 5x
x - 5x = -20
-4x = -20
x = 5
2) 20*7 + 7x = 287
140 + 7x = 287
7x = 287 - 140
7x = 147
x = 21
3) 2(x + x + 10) = 208
2x + 2x + 20 = 208
4x + 20 = 208
4x = 208 - 20
4x = 188
x = 47
