Set your expression for the cost equal to 87, since Mason’s total charge was $87. What equation do you get?

First, subtract the flat fee from both sides of the equation to find how much of the $87 was from mileage alone.

You should get an equation like $0.20m = \text{(some number)}$. How do you undo multiplying by 0.20 to solve for $m$?

In Desmos, type the expression (87 - 55) / 0.2 and look at the numerical result. That value is the number of miles Mason drove.

Let $m$ be the number of miles Mason drove.

The company charges a flat fee of $55 plus $0.20 for each mile, so the total charge is modeled by:

This equation says: flat fee + (cost per mile)$\times$(miles) = total cost.

To get the term with $m$ by itself, subtract 55 from both sides of the equation:

Calculate the right side: $87 - 55 = 32$, so

Now divide both sides of the equation by $0.20$ to solve for $m$:

Since $0.20 = \frac{1}{5}$, dividing by $0.20$ is the same as multiplying by 5:

So $m = 160$, meaning Mason drove 160 miles.