`240 : (x - 5) = 2^{2} . 5^{2} - 20`
$\Rightarrow$ `240 : (x - 5) = 4 . 25 - 20`
$\Rightarrow$ `240 : (x - 5) = 100 - 20`
$\Rightarrow$ `240 : (x - 5) = 80`
$\Rightarrow$ `(x - 5) = 240 : 80`
$\Rightarrow$ `x - 5 = 3`
$\Rightarrow$ `x = 3 + 5`
$\Rightarrow$ `x = 8`
$\text{Vậy}$ `x = 8`