Example
Zoe is the only lemonade seller on her street. People who drive home stop by to purchase liters of her lemonade.
The price for lemonade follows $$P = 10 - 2 Q$$
If Zoe produces Q=2 liters of lemonade, the price is $6: $$P = 10 - 2 Q = 10 - 2 \times 2 = 10 - 4 = 6$$
Question
The price for lemonade follows $$P = 21 - 5 Q$$ At what price will Zoe sell `Q=2` liters of lemonades?
Plug `Q=2` into `P = 21 - 5 Q` \begin{align*} P &= 21 - 5 Q \\ P &= 21 - 5 \times 2 \\ P &= 21 - 10 \\ P &= 11 \end{align*} The price of a liter of lemonade is $11.