`0,4 : 0,12 = x : 0,9`

`<=> 10/3 = x : 0,9`

`<=> x : 0,9 = 10/3`

`<=> x = 10/3  . 0,9`

`<=> x = 3`

Vậy `x = 3`

`---`

`13 1/3 : 1 1/3 = 26 : ( 2x - 1 )`

`<=> 40/3 : 4/3 = 26 : ( 2x - 1 )`

`<=> 10 = 26 : ( 2x - 1 )`

`<=> 26 : ( 2x - 1 ) = 10`

`<=> 2x - 1 = 26 : 10`

`<=> 2x - 1 = 2,6`

`<=> 2x = 2,6 + 1`

`<=> 2x = 3,6`

`<=> x = 3,6 : 2`

`<=> x = 1,8`

Vậy `x = 1,8`

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