This was CS50x Puzzle Day 2020

David J. Malan

CS50x Puzzle Day is an online adaptation of an event we hold at Harvard University each year, an opportunity for students around the world to collaborate on a team with classmates, family, and friends on a packet of puzzles, which are essentially logic (not jigsaw!) problems. Whereas the on-campus event is just a single day, the online adaptation spans four days, Friday through Monday, thereby allowing participants to collaborate with classmates or colleagues (on Friday or Monday) or with family and friends (on Saturday or Sunday). Previously held in 2016, 2017, 2018, and 2019, this year’s event ended just a few hours ago, having started at 00:00:00 on Friday, 3 April 2020, and having ended at 23:59:59 on Monday, 6 April 2020.

Of course, we didn’t expect everyone to be home for this year’s event. Indeed, when we announced this year’s event on 21 February 2020, the world was a healthier place. Indeed, as more of the world took ill, we considered postponing this year’s event, particularly since the tradition is as much about solving problems as it is about bringing folks together in person to collaborate, as so many photos from years past attest. Comments online like Lucie’s, though, ultimately convinced us otherwise:

But we began to message that this year’s event would be online-only:

CS50x Puzzle Day 2020 shall be online-only this year. Team members should NOT gather in person, unless they already live together. All team members should instead collaborate online via chat, voice, or video. Doctors’ orders!

Teams discovered to have gathered in person will be disqualified and ineligible for certificates.


Ultimately, 13,427 people around the world pre-registered for this year’s event, up from 8,994 last year (+49%), 11,020 (82%) of whom would be participating in CS50x Puzzle Day for the very first time. And those registrants hailed from 167 countries. Among the most represented, each with at least 100 registrants, were:

  1. United States
  2. India
  3. Ukraine
  4. Iran
  5. United Kingdom
  6. Canada
  7. Pakistan
  8. Egypt
  9. Brazil
  10. Germany
  11. Vietnam
  12. Singapore
  13. Nigeria
  14. Australia
  15. Algeria
  16. Iraq
  17. Malaysia
  18. France
  19. Netherlands

Here’s the packet of puzzles that everyone was given, wonderfully written by Facebook’s own Matthew Warshauer ’14, a former CS50 student!

By event’s end, approximately 2,300 teams collectively comprising 6,184 members had submitted their answers. (At least a few more, we suspect, were perhaps too shy to submit!) And 299 teams solved all 8 out of 8 puzzles!

Nearly all teams (2,068) submitted a correct answer Secret Message from US, while 114 teams submitted an incorrect answer for the same. Here’s how the teams fared on each of the puzzles:

A favorite among teams was A Poem for Hannah:

However, among teams who solved all 8 out of 8 puzzles, Symbolism was actually the favorite.

A Secret Message from US seems to have been the easiest puzzle by far:

Meanwhile, Sword Search appears to have been the hardest!

Teams appear to have followed doctors’ orders, collaborating with teammates online, with WhatsApp the most popular tool of choice:

And now the answers to the puzzles themselves. Spoilers ahead!

  • Symbolism: GABON or LIBREVILLE
  • Secret Message from US: QWERTY
  • Sets: ADDITION or ADD
  • Sword Search: CROSS SWORDS
  • Putting It All Together: SPILSBURY
  • A Poem for Hannah: AIBOHPHOBIA
  • Stretch Out and Break Up to Get In: LOCKSMITH

And here’s how each team scored! Congratulations to all! Everyone whose team solved at least 5 (a majority) of the 8 puzzles was awarded a CS50 Certificate.

Not sure how to solve one (or more!) of the puzzles? Here are walkthroughs of each, thanks to CS50’s own Brian Yu.


Thanks to everyone around the world who took photos (and screenshots) of their teams solving puzzles!


And thanks to everyone around the world who created memes!



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