Norah invited 4 friends to a table tennis party. Each of
the 5 people at the party played every other person
exactly 1 time. The table below shows the number of
games won by each player except Norah. There were
no ties. How many games did Norah win?
$$ \begin{array}{|c|c|} \hline
\text{Player} & \text{Games won} \\ \hline
\text{Collier} & 2 \\
\text{Evangeline} & 1 \\
\text{Gabe} & 1 \\
\text{Norah} & ? \\
\text{Maria} & 3 \\ \hline
\end{array} $$
We can find the number of games played. We are looking for the number of ways to form unique pairs out of 5 people.
Order does not matter.
$$ {}_nC_r = \frac{n!}{(n-r)!r!} $$
$$ {}_5C_2 = \frac{5!}{(5-2)!2!} $$
$$ {}_5C_2 = \frac{5!}{3!2!} $$
$$ = \frac{5\cdot 4\cdot 3\cdot 2 }{3\cdot 2 \cdot 2} $$
$$ = 10 $$
Norah must have won:
$$ 10-2-1-1-3 = \boxed{3} $$
We can figure out the total number of games through inspection.
Starting at Collier, Collier must have played 4 games to play with everyone else.
Evangeline will play 4 games with everyone else, but 1 will be a duplicate with Collier, so we have \(4+3\) games.
Gabe plays 4 games, but 2 have already been mentioned, so the total games is \(4+3+2\).
Continuing the pattern...
$$ \text{Games played} = 4+3+2+1 = 10 $$
Each game had a winner, so there are 10 games won.
$$ 2+1+1+?+3 = 10 $$
$$ ? = \boxed{3} $$