Since $2$ cups of lemonade is twice as much as $1$ cup of lemonade, to find the price of $1$ cup of lemonade, you must find half of the price of $2$ cups of lemonade.

You can also solve the problem by setting up a proportion like the one below, then solve for $x$.

$\large\frac{\text{2 cups of lemonade}}{\text{3.60 dollars}} \left( \frac{?}{?} \right) = \frac{\text{1 cup of lemonade}}{x\text{ dollars}}$

To find the price of any amount of lemonade, in the equation above substitute ''$1$ cup of lemonade'' with the number of lemonades you are looking for.

$\begin{array}{c|c} {\quad \;\text{# of} \\ \text{Lemonades}} & {\; \\ \quad \text{Price (\$)} \quad } \\ \hline 1 & \mathbf{1.80}\\ 2 & 3.60 \\ 3 & \mathbf{5.40}\\ 4 & \mathbf{7.20}\\ 5 \\ 6 \\ 7 \\ 8 \end{array}$