1000 - (x ⋅ 8 - 20) = 300
x ⋅ 8 - 20 = 1000 - 300
x ⋅ 8 - 20 = 700
x ⋅ 8 = 700 + 20
x ⋅ 8 = 720
x = 720 : 8
x = 90
Ответ: 90