Determine the number of integer solutions to $x_1+x_2+x_3+x_4 \le 72$ such that:
- $1 \le x_1 \le 12$
- $0 \le x_2\le 10$
- $3 \le x_3\le 13$
- $5 \le x_4\le 36$
At first, I introduced a variable $x_5$ such that $x_1+x_2+x_3+x_4+x_5=72$ and applied a linear transformation such that:
- $y_1 = x_1-1$
- $y_2 = x_2$
- $y_3 = x_3-3$
- $y_4 = x_4-5$
- $y_5 = x_5$
and then regarranging and substituting values into the original equation, I get $y_1+y_2+y_3+y_4+y_5=63$ such that:
- $y_1 \le 12$
- $y_2 \le 10$
- $y_3 \le 13$
- $y_4 \le 36$
- $y_5 \ge 0$
my instincts tell me I should apply another linear transformation, but I'm not quite sure how to go from here.
Any tips would be appreciated!