if we take the 850 to be the 100%, then how much is 6.5% of that?
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
850&100\\
t&6.5
\end{array}\implies \cfrac{850}{t}=\cfrac{100}{6.5}\implies \cfrac{850\cdot 6.5}{100}=t[/tex]
so the tax is "t", add that to the 850 sale price, and the total price is 850 + t.