fix 15 in 4.9

src/Epp.tex

@@ -15598,14 +15598,14 @@ \subsubsection{Exercise 15}
 
 There are 3 people who network with 6 other people: in other words there are 3 vertices of degree 6. Similarly, one vertex of degree 5, 5 vertices of degree 4, and say $x$ vertices of degree 3 (let $x$ be the number of people who are network friends with 3 other people). And there are a total of 41 edges.
 
-The total degree is $42 \cdot 2 = 82$. Counted the other way, the total degree is: $3 \cdot 6 + 1 \cdot 5 + 5 \cdot 4 + 3x = 43+3x$. So $82 = 43+3x$, therefore $39 = 3x$ so $x = 13$. So 13 people are network friends with three other people in the network.
+The total degree is $41 \cdot 2 = 82$. Counted the other way, the total degree is: $3 \cdot 6 + 1 \cdot 5 + 5 \cdot 4 + 3x = 43+3x$. So $82 = 43+3x$, therefore $39 = 3x$ so $x = 13$. So 13 people are network friends with three other people in the network.
 \end{proof}
 
 (b)
 How many people are in the network?
 
 \begin{proof}
-$4 + 1 + 5 + 13 = 23$ people.
+$3 + 1 + 5 + 13 = 22$ people.
 \end{proof}
 
 \subsubsection{Exercise 16}