# PSE Advent Calendar 2023 (Day 3): 24 dangerous doors

This puzzle is part of the Puzzling Stack Exchange Advent Calendar 2023. The accepted answer to this question will be awarded a bounty worth 50 reputation.

< Previous Door Next Door >

Task: Solve the killer sudoku

• Normal sudoku rules apply
• In cages, digits must sum to the small clue on the top left corner of the cage. Digits cannot repeat in a cage.

Solve on Penpa+

• Having seen the solution below, I still don't understand "dangerous". Dec 3, 2023 at 12:37
• @msh210 I think that it's just a reference to the "killer" part. I'm impressed that the puzzle uses precisely the numbers 1-24 exactly once each in order to resemble an advent calendar. Dec 3, 2023 at 12:45
• Yes, I am , too. Dec 3, 2023 at 12:50
• Ooh, I didn't realize that! That's very clever! Dec 3, 2023 at 12:58
• That explains the "doors". Dec 3, 2023 at 19:40

## 1 Answer

Solved grid:

It was fun!

General solve path:

Start with the obvious one box clue R3C5 and R8C7, then to the 3-sum on C9. Then the 6-sum on top right box, which must be 1+2+3. The 4-sum on C1 is also solved.
Some general strategy is mostly to consider the possible numbers on a cage to get some sure possibilities (e.g., must be 5+7 instead of 4+8), and to consider that total sum in a 3x3 box is 45 (e.g., the mid right box, being covered with all sums, helped to deduce the 22-cage should have a sum of 13 in that box, so can put 9 in R6C6).
The longest stuck point was resolving the 20-cage on R3. Eventually I figured that it's either 4 or 9 in R4C1, but dunno which. But figured that 4 will lead to a 7+9 in the remaining two spots, which leaves only 4 as the valid candidate on both R3C6 and R3C7, a contradiction. After getting past that, it's easy again.