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`a)` `1/4=20/x`

`<=>` `x=20.4`

`<=>` `x=80`

Vậy `x=80`

`b)` `{-3}/x=6/10`

`<=>` `x=-3:3/5`

`<=>` `x=-3.{5}/3`

`<=>` `x=-5`

Vậy `x=-5`

`c)` `{12-x}/{-8}=15/6`

`<=>` `12-x=5/{2}.(-8)`

`<=>` `12-x=-20`

`<=>` `x=12+20`

`<=>` `x=32`

Vậy `x=32`

`d)` `{x+3}/{-27}={-3}/{x+3}`

`<=>` `(x+3)(x+3)=-27.(-3)`

`<=>` `(x+3)^2=81`

`<=>` `(x+3)^2=(±9)^2`

`<=>` $\left[\begin{matrix} x+1=8\\ x+3=-8\end{matrix}\right.$

`<=>` $\left[\begin{matrix} x=7\\ x=-11\end{matrix}\right.$

Vậy `x∈{-11;7}`