Let \(x\) represent the number of quarts you can buy for $10.00, or 1000 cents. Because the variables total price and number of quarts are proportional, we know that their ratio is constant. Thus, the ratio
\begin{gather*}
\dfrac{\text{1000 cents}}{x~\text{quarts}}~~~~~~\text{is equal to the ratio}~~~~~~\dfrac{\text{80 cents}}{\text{1 quart}}\\
\dfrac{1000}{x}=\dfrac{80}{1}
\end{gather*}
We solve the proportion by cross-multiplying to get
\begin{align*}
1000(1) \amp = 80x \amp \amp \blert{\text{Divide both sides by 80.}}\\
x \amp = \dfrac{1000}{80}=12.5
\end{align*}
You can buy 12.5 quarts of grape juice for $10.00. (Or, if you can only buy quart bottles, then you can buy 12 quarts and have 40 cents left over.)