College Algebra, Chapter 1, 1.1, Section 1.1, Problem 98

Suppose that a toy maker finds that it costs $c = 450 + 3.75x$ dollars to manufacture $x$ toy trucks. If the budget allows \$3600 in costs, how many trucks can be made?

Let $x$ be the number of toy trucks and $c$ is the cost to manufacture toy trucks.
If the budget allows \$3600, then
$c = \$3600$

So,

$
\begin{equation}
\begin{aligned}
c &= 450 + 3.75x \\
\\
3600 &= 450 + 3.75x && \text{Substitute value of } c\\
\\
3600-450 &= 450 +3.75x -450 && \text{Subtract both sides by 450}\\
\\
\frac{3150}{3.75} &= \frac{\cancel{3.75}x}{\cancel{3.75}} && \text{Divide both sides by 3.75}\\
\\
x&= 840
\end{aligned}
\end{equation}
$


840 toy trucks can be manufacture in the allowed budget.