An architect is building a model of a tennis court for a new client. On the model, the court is 6 inches wide and 13 inches long. An official tennis court is 36 feet wide. What is the length of a tennis court?

The court is $78$ feet long
We can make a ratio out of this problem. We know that $6$ and $36$ are proportional to eachother, which gives us $\frac{6}{36} = \frac{13}{l}$. To solve for $l$, we just simplfy $\frac{6}{36}$ to $\frac{1}{6}$. That gives us $\frac{1}{6} = \frac{13}{l}$. Now we just multiply both sides by $6$, which looks like this $\cancel{6} \cdot \frac{1}{\cancel{6}} = \frac{13}{l} \cdot 6$. If we multiply both sides by $l$, we get $l = 78$.