To solve the exercise you can use a rule of three, like this
[tex]\begin{gathered} \text{ \$78.99}\rightarrow100\text{\%} \\ \text{ \$}x\rightarrow5.75\text{\%} \\ x=\frac{5.75\text{\%}\cdot\text{ \$78.99}}{100\text{\%}} \\ x=\text{\$}\frac{5.75\cdot78.99}{100} \\ x=\text{\$}\frac{454.19}{100} \\ x=\text{\$}4.54 \end{gathered}[/tex]The shipping fee would be $4.54 which corresponds to 5.75% of the total purchase. But since $4.54 is less than $4.95 then Lucia will pay for shipping $4.95.
