CS50x Puzzle Day 2025

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What is CS50x Puzzle Day?

Solve a packet of problems!

CS50x Puzzle Day is an online adaptation of an event we hold at Harvard each year, an opportunity to collaborate on a team with classmates, family, friends, students, and colleagues on a packet of puzzles (i.e., logic problems). The event is open to everyone around the world, whether taking CS50 or not. No prior CS experience required. Teams of size 2, 3, or 4 are encouraged, or you can participate on your own. Teachers are welcome to host their own version during class.

Starts anytime after 00:00 on Friday, April 4, 2025, in your own time zone. Ends anytime before 23:59 on Monday, April 7, 2025, in your own time zone.

Register at cs50.ly/register anytime before 23:59 on Monday, April 7, 2025, in your own time zone. Every member of your team should register individually by submitting the form as well.

See cs50.medium.com/this-was-cs50x-puzzle-day-2024-1de7a4bbce0c for a summary of last year’s CS50x Puzzle Day as well as some past puzzles for practice. Indeed, the best way to practice for this year is to solve past puzzles first!

How does it work?

  • Anytime after 00:00 on Friday, April 4, 2025, in your own time zone, your team will download a packet of puzzles as a PDF from cs50.ly/puzzles.
  • Your team will then have until 23:59 on Monday, April 7, 2025, in your own time zone, to solve as many puzzles as you can. You’ll submit your answers online via a Google Form at cs50.ly/submit.
  • Take photos while you work on the puzzles (or screenshots of your team collaborating online)! We’ll then share them online if you’d like.
  • After the event ends, we’ll hold a live walkthrough of the puzzles’ answers via Zoom. If unable to attend live, you can also watch on demand afterward.
  • CS50 certificates will be awarded to the teams that answer the most questions correctly.
  • Puzzles will not require prior programming experience, just logic and smarts. All you’ll need is a computer with internet access, a pen or pencil, and scratch paper.

What are the rules?

  • Teams of size 2, 3, or 4 are encouraged, but you may form larger teams of reasonable size. (You’ll declare your team when you submit your answers.) Consider, though, that the larger your team, the less each team member will be challenged by puzzles! Teams deemed by CS50’s staff to be of unreasonable size will be disqualified and ineligible for certificates.
  • Teams may not share hints or answers with other teams. Teams deemed by CS50’s staff to have shared hints or answers with other teams will be disqualified and ineligible for certificates.

See CS50x Puzzle Day 2019’s discussion of honesty for context!

What are the puzzles?

Coming 00:00 on Friday, April 4, 2025!

Errata

We’ll announce any errata (mistakes) here during the event!

Photos (and memes)!

Take photos while you work on the puzzles (or screenshots of your team collaborating online)! Or create CS50x Puzzle Day-related memes! Share them at cs50.ly/photos if you’d like!

How to practice?

The best way to practice for this year is to solve some past puzzles first!

Looking for teammates?

Post in any of CS50’s communities if looking to form a team! Perhaps share a little bit about yourself in your post.

How to host your own?

You’re welcome to adopt or adapt CS50x Puzzle Day for your own students or organization, per CS50’s license! Allow us to suggest a few things to consider as you plan ahead, and always feel free to get in touch at outreach@cs50.harvard.edu!

Sample Timeline

Though CS50x Puzzle Day is hosted over a few days, we find that, in-person, a single day suffices! Here’s a sample timeline of our own Puzzle Day on campus:

Time Event
11:45am Doors open
11:45-12:15 Lunch
12:15-12:30 Instructions
12:30-2:45 Puzzle solving; solution packet in by 2:45 on the dot!
2:45-3:00 Solutions revealed
3:00–3:30 Puzzle walkthroughs

Food

We typically order pizza assuming 3 slices per person. Sometimes that overshoots it, sometimes it’s just right. We also usually order one large-sized veggie platter for every 20 or so pizzas.

Setting the Tone

Be sure to encourage that “winning” isn’t the goal – problem solving and creative thinking is the goal. Having fun is the most important thing. If giving out prizes to winning team(s), we suggest also setting up raffle prizes and minimally message the element of prizes to ensure most people don’t attend expecting or looking for a prize.

Solution Reveals

Solutions to the puzzles won’t be released until after 23:59 on Monday, April 7, 2025, so you may want to host your event after CS50x Puzzle Day itself, or reveal the solutions at a separate event!

Supplies

You don’t need much to run CS50x Puzzle Day! We find that pen, paper, and computers with internet are more than sufficient. Consider having tables for groups to work together! In more recent times, keeping masks and other PPE handy is definitely a must. Be sure to follow the COVID-19 guidelines of your local area.

Quotes from past CS50x Puzzle Days!

  • “A fun mental workout.”
  • Be prepared for a brain-scratching joyride
  • Perfect opportunity to challenge yourself and test your problem solving skills!
  • A great challenging event for puzzle-loving types around the world!
  • Every step towards the answer is like a massive lightbulb “DING!”
  • Fun and Challenging!
  • Attempting the Puzzle Day Packet with two of my friends ended up being a favourite moment of ours together.
  • Perfect opportunity to challenge yourself and test your problem solving skills!

Questions?

If you have a question about a puzzle, email puzzles@cs50.harvard.edu, and we’ll do our best to reply! Afraid, though, we won’t be able to offer any hints or tips!

How to submit?

Submit your answers at cs50.ly/submit by 23:59 on Monday, April 7, 2025, in your own time zone!

Who are the winners?

To be announced after 2025-04-08T13:00:00-04:00!

What are the right answers?

To be announced via Zoom on 2025-04-08T13:00:00-04:00!

Results

To be announced via Zoom on 2025-04-08T13:00:00-04:00!