Look, you can't complain about this after giving us so many scenarios involving N locked chests and M unlabeled keys.
D&D Combinatorics [Explained]
Transcript
[Cueball, Megan, Ponytail, White Hat, and Knit Cap are sitting at a table. Everyone is looking at Cueball. Ponytail is facepalming. The table is covered in sheets of paper and assorted dice.] Cueball: I grab 2 of the 10 arrows without looking and fire them, hoping I didn't grab one of the 5 cursed ones. Did I? Ponytail: Sigh. Umm. Okay. Ponytail: Roll... Uh... Hang on... Ponytail: Roll 3d6 and a d4. You need... 16 or better to avoid the cursed arrows.
[Caption below the panel:] I got way more annoying to play D&D with once I learned that our DM has a combinatorics degree and can't resist puzzles.