`(1 - 3/10 - x) : (19/10 - 1 - 2/5) + 4/5 = 1`

`(10/10 - 3/10 - x) : (19/10 - 10/10 - 4/10) = 1 - 4/5`

`(7/10 - x) : 1/2 = 1/5`

`7/10 - x = 1/5 . 1/2`

`7/10 - x = 1/10`

`x = 7/10 - 1/10`

`x = 6/10`

`x = 3/5`

Vậy `x = 3/5`

Đáp án:

 ( 1 - $\dfrac{3}{10}$ - x ) : ( $\dfrac{19}{10}$ - 1 - $\dfrac{2}{5}$ ) + $\dfrac{4}{5}$ = 1

 ( $\dfrac{10}{10}$ - $\dfrac{3}{10}$ - x ) : ( $\dfrac{19}{10}$ - $\dfrac{10}{10}$ - $\dfrac{4}{10}$ ) + $\dfrac{4}{5}$ = 1

 ( $\dfrac{7}{10}$ - x ) : $\dfrac{19-10-4}{10}$ + $\dfrac{4}{5}$ = 1

 ( $\dfrac{7}{10}$ - x ) : $\dfrac{1}{2}$ + $\dfrac{4}{5}$ = 1

 ( $\dfrac{7}{10}$ - x ) : $\dfrac{1}{2}$ = 1 - $\dfrac{4}{5}$ 

 ( $\dfrac{7}{10}$ - x ) : $\dfrac{1}{2}$ =  ( $\dfrac{1}{5}$

   $\dfrac{7}{10}$ - x  =  $\dfrac{1}{5}$ x $\dfrac{1}{2}

  $\dfrac{7}{10}$ - x = $\dfrac{1}{10}

   x =   $\dfrac{7}{10}$ - $\dfrac{1}{10}

   x =       $\dfrac{6}{10}

   x =       $\dfrac{3}{5}

Vậy    x = $\dfrac{3}{5}