Question: 2015 Fall Midterm - 9
How many solutions are there to the equation $x_1 + x_2 + x_3 + x_4 = 55$,
where $x_1 \geq 0$, $x_2 \geq 0$, $x_3 \geq 0$, and $x_4 \geq 0$ are integers?
(a)
${59 \choose 3}$
(b)
${58 \choose 3}$
(c)
${58 \choose 4}$
(d)
${59 \choose 4}$