Tìm x , biết : 280 - X × 9 - x = 80

`280 - x × 9 - x = 80`

`=> 280 - (x × 9 + x) = 80`

`=> 280 - x × (9 + 1) = 80`

`=> 280 - x × 10 = 80`

`=> x × 10 = 280 - 80`

`=> x = 200 : 10`

`=> x = 20`

Vậy `x = 20`