Đáp án `+` Giải thích các bước giải `:`

`1)` `2/5 + 3/5x = 9/20`

`=>` `3/5x = 9/20 - 2/5`

`=>` `3/5x = 1/20`

`=>` `x = 1/20 : 3/5`

`=>` `x = 1/12`

Vậy `x = 1/12`

``

`2)` `1/4 + 1/3 : (3x) = -5`

`=>` `1/4 + 1/3 . (1/3x) = -5`

`=>` `1/4 + 1/9x = -5`

`=>` `1/9x = (-5) - 1/4`

`=>` `1/9x = -21/4`

`=>` `x = 1/9 : (-21/4)`

`=>` `x = -4/189`

Vậy `x = -4/189`

``

`3)` `-21/13x + 1/3 = -2/3`

`=>` `-21/13x = (-2)/3 - 1/3`

`=>` `-21/13x = -1`

`=>` `x = -1 : (-21/13)`

`=>` `x = 13/21`

Vậy `x = 13/21`

``

`4)` `3/4x - 1/2 = 3/7`

`=>` `3/4x = 3/7 + 1/2`

`=>` `3/4x = 13/14`

`=>` `x = 13/14 : 3/4`

`=>` `x = 26/21`

Vậy `x = 26/21`