( x - 1) + ( x - 2 ) + ( x - 3 ) + ( x - 4 ) = 50
$\Rightarrow$ x - 1 + x -2 + x - 3 + x - 4 = 50
$\Rightarrow$ ( x + x + x + x ) - ( 1 + 2 + 3 + 4 ) = 50
$\Rightarrow$ 4x - 10 = 50
$\Rightarrow$ 4x = 50 + 10
$\Rightarrow$ 4x = 60
$\Rightarrow$ x = 60 : 4
$\Rightarrow$ x = 15
Vậy x = 15