Question: 2015 Winter Final - 11
How many solutions are there to the equation $x_1 + x_2 + x_3 + x_4 = 99$, where $x_1 \geq 0$,
$x_2 \geq 0$, $x_3 \geq 0$, $x_4 \geq 0$ are integers?
(a)
${102 \choose 4}$
(b)
${103 \choose 4}$
(c)
${102 \choose 3}$
(d)
${103 \choose 3}$