well, the error value is 8, he was off by 8 units... so, if we take 68 to be the 100%, what is 8 in percentage off of it?
[tex]\bf \begin{array}{ccllll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
68&100\\
8&x
\end{array}\implies \cfrac{68}{8}=\cfrac{100}{x}[/tex]