We shall consider the options one after the other before choosing the best option;
Option 1;
[tex]\begin{gathered} \text{Pack of 8 costs 3.12} \\ 1\text{ unit = }\frac{3.12}{8} \\ 1\text{ unit=0.39} \end{gathered}[/tex]
Option 2;
[tex]\begin{gathered} \text{Pack of 15 costs 6.15} \\ 1\text{ unit=}\frac{6.15}{15} \\ 1\text{ unit=0.41} \end{gathered}[/tex]
Option 3;
[tex]\begin{gathered} \text{Pack of 20 costs 7.60} \\ 1\text{ unit=}\frac{7.60}{20} \\ 1\text{ unit=0.38} \end{gathered}[/tex]
Based on the calculations above, the third option is the best deal because that would make you pay the least amount per underwear. By choosing option 3, you will end up paying $0.38 for each underwear.
Therefore, "a package of 20 costs $7.60" is the best buy.
