$x_1+x_2+x_3+x_4 = 100$ with
$1 \le x_1\le10$
Apparently this is the same as
$y_1 + y_2 + y_3 + y_4 = 92$ with
$y_1 \le 9$
I understand the $3$ conditions, but what has happened to $x_3$? I can see that it has been subtracted but what has happened to the conditions?