Two quantities are proportional if the ratio between those quantities is always the same.
Find the ratio of price/number of books for each row in the table:
[tex]\begin{gathered} \frac{3}{1}=3 \\ \frac{9}{3}=3 \\ \frac{12}{4}=3 \\ \frac{18}{7}=2.57\ldots \end{gathered}[/tex]
We can see that the ratio price:books is 3 if the number of books is 1, 3 or 4 but it is a different ratio when the number of books is 7.
Therefore, the price is not proportional to the number of books.
Sidenote:
If the number of books on the last row was 6 instead of 7, then the ratio would still be 3 and the price would be in fact proportional. Make sure that it is not a printing error of the problem sheet.
