Answer
a) $C(n)=\frac{1920+100n}{n}$
b) about $5$ people
Work Step by Step
a) Since each pizza contains $960$ calories and each person eats a banana which has $100$ calories, then the calories consumed with $2$ pizzas and $n$ bananas by $n$ people is
$$\begin{aligned}
C(n)&=\frac{2(960)+100n}{n}
\\&=
\frac{1920+100n}{n}
.\end{aligned}$$
The graph of $C(n)=\frac{1920+100n}{n}$ is shown below.
b) The line $y=500$ intersects the graph at approximately $(4.8,500)$. Hence, approximately $5$ people must share the pizza for each person to consume about $500$ Calories.
