A **proportion** is an equation indicating two ratios are equal.

If *a* is to *b* as *c* is to *d*, then $\genfrac{}{}{0.07ex}{}{a}{b}=\genfrac{}{}{0.07ex}{}{c}{d}$.

If $\genfrac{}{}{0.07ex}{}{a}{b}=\genfrac{}{}{0.07ex}{}{c}{d}$, then, using cross multiplication,
$ad=bc$.

Set up a proportion to represent the situation.

Use a variable to represent the unknown value.

$\genfrac{}{}{0.07ex}{}{16\text{passengers}}{2\text{cars}}=\genfrac{}{}{0.07ex}{}{p}{5\text{cars}}$

Cross multiply, then solve.

$\begin{array}{rcl}\text{(}16\text{passengers}\text{)}\text{(}5\text{cars}\text{)}& =& \text{(}p\text{)}\text{(}2\text{cars}\text{)}\\ 80\text{passengers \xb7 cars}& =& 2p\text{cars}\\ 40\text{passengers}& =& p\end{array}$

So, 40 passengers can be on the track at one time.