Assuming that Al had no model cars before he started to buy a new one each week, then, after N weeks, Al will own N cars.
If we divide the number of cars by the number of weeks, it will always equal a constant of proportionality of 1, since:
[tex]\begin{gathered} \frac{\text{cars}}{\text{weeks}}=\frac{N}{N} \\ =1 \end{gathered}[/tex]Therefore, the number of model cars he owns is proportional to the number of weeks that have passed since he started his collection.
