In the casework method, \(10+9+8+...+1\) looks very familiar... it's the famous shaking hands problem!
Suppose you have 11 people shaking hands with each other, then it's \(10+9+...+1\) I'm pretty sure no explanation is needed.
How about the stars ⭐ and bars 📊 method? It turns out you can explain it using the more "fancy" trick too!
The number of ways to choose 2 out of 11 people to shake hands with each other is, well, \(\binom{11}{2}\).
Same as the stars and bars grade curve problem!
And if there are 4 grades, like in the mini-question...
You will have 3 people shaking hands with each other at the same time!
Weird, but true.