Example 1.6.3.
Earlier (Example 1.5.3) we counted the number of solutions to the equation
\begin{equation*}
x_1 + x_2 + x_3 + x_4 + x_5 = 13\text{,}
\end{equation*}
where \(x_i \ge 0\) for each \(x_i\text{.}\)
How many of those solutions have \(0 \le x_i \le 3\) for each \(x_i\text{?}\)