`a,`

`61 - 3 . x = 31`

`=> 3 . x = 61 - 31`

`=> 3 . x = 30`

`=> x = 30 : 3`

`=> x = 10`

Vậy `x = 10`

`b,`

`100 - 3 . x = 58`

`=> 3 . x = 100 - 58`

`=> 3 . x = 42`

`=> x = 42 : 3`

`=> x = 14`

Vậy `x = 14`

`c,`

`123 - 25 . x = 23`

`=> 25 . x = 123 - 23`

`=> 25 . x = 100`

`=> x = 100 : 25`

`=> x = 4`

Vậy `x = 4`

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