Answer
$0.00078125$
Work Step by Step
Substituting $x_T=0.2$ and $x=\dfrac{51}{256}$ in the expression $|x_T-x|$, then the absoslute error is
\begin{array}{l}\require{cancel}
\left| 0.2-\dfrac{51}{256} \right|
\\\\=
\left| 0.2-0.19921875 \right|
\\\\=
\left| 0.00078125 \right|
\\\\=
0.00078125
.\end{array}