Question: 2014 Fall Final - 9
How many solutions are there to the equation $x_1 + x_2 + x_3 = 13$, where $x_1 \geq 0$, $x_2 \geq 0$,
$x_3 \geq 0$ are integers?
(a)
${13 \choose 2}$
(b)
${16 \choose 2}$
(c)
${15 \choose 2}$
(d)
${14 \choose 2}$