(980 : x) ⋅ 18 - 90 = 270
(980 : x) ⋅ 18 = 270 + 90
(980 : x) ⋅ 18 = 360
980 : x = 360 : 18
980 : x = 20
x = 980 : 20
x = 49
Ответ: 49