well, if the selling price was 210, and that's the 100%, then if he adds 60% to that, the new price will be 160% of 210... well, if 210 is the 100%, what's 160% of that?
[tex]\bf \begin{array}{ccllll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
210&100\\
x&160
\end{array}\implies \cfrac{210}{x}=\cfrac{100}{160}[/tex]
solve for "x".
