The following is a geometry puzzle from a math school book. Even though it has been a long time since I finished school, I remember this puzzle quite well, and I don't have a nice solution to it.
So here is the puzzle:
The triangle $ABC$ is known to be isosceles, that is, $AC=BC$. The labelle...
Say I want to find the number of nonnegative integer solutions to
$$x_1+x_2+x_3+x_4=22$$
How do I apply stars and bars for this?
What if there is an inequality or the lower bounds on the $x_i$ are not zero?