ANSWER:
17
STEP-BY-STEP EXPLANATION:
We can calculate the number of people who had pizza using the following equation:
Let x be the number of people who had pizza:
[tex]\begin{gathered} \frac{1}{7}\cdot28+\frac{1}{4}\cdot28+x=28 \\ \text{ solving for x} \\ 4+7+x=28 \\ x=28-4-7 \\ x=17 \end{gathered}[/tex]The number of people who had pizza was 17