This was CS50x Puzzle Day 2018

David J. Malan

26 min readMar 14, 2018

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First came CS50x Puzzle Day 2016. Then came CS50x Puzzle Day 2017. Then, just this past weekend, came CS50x Puzzle Day 2018, the third-ever adaptation of CS50’s on-campus tradition for students online. At 00:00:00 on Friday, 9 March 2018, students around the world were invited to download a packet of puzzles, written by CS50’s own Doug Lloyd and Facebook’s own Matthew Warshauer:

Students were encouraged to form teams of size two or more, collaborating in person or online, though students could also participate solo. In addition to a pen or pencil (and scratch paper!), students were welcome to use a computer (and the internet), though, as always, none of the puzzles required prior programming experience, just logic and smarts.

3,274 students pre-registered (via a Google Form) for the event, only 12.1% of whom had participated in CS50x Puzzle Day 2017, and only 4.2% of whom had participated in CS50x Puzzle Day 2016. 540 students sought teammates via the event’s “teammate finder” (a separate Google Form), and 77 students offered to host the event for classmates nearby.

Among the registrants were

762 students from Ukraine,
639 students from the United States,
557 students from India,
117 students from Egypt,
94 students from Great Britain,
79 students from Canada,
60 students from Pakistan,
44 students from Netherlands,
43 students from Russia,
42 students from Germany,

along with students from 109 other countries.

The goal for each team? Solve as many of the puzzles as possible by 23:59:59 on Monday, 12 March 2018.

And so many teams did! Indeed, 826 teams solved at least one puzzle correctly, and 108 teams even solved all eight:

Here’s how everyone scored. Each team that scored 8/8 will receive a CS50x Puzzle Day 2018 Certificate, and a pseudorandom subset will additionally receive some swag from our friends at Facebook!

If you were on a team that scored 8/8, be sure to submit this form! And congrats!

Ultimately, 1,300 teams comprising 2,476 students competed in CS50x Puzzle Day 2018 (and even more might have participated without using Facebook). Collectively, those teams submitted 20,461 attempts at answers.

As for the puzzles themselves, the most challenging was perhaps #7 Medal Table, with only 117 teams submitting a correct answer (and 333 others submitting attempts), and the easiest was perhaps #1 Logic Gate, with 780 teams submitting a correct answer (and 66 others submitting attempts):

For each puzzle, number of teams that correctly answered (in green) and number of teams that attempted to answer (in red).

Now, CS50x Puzzle Day 2018 itself would not have been possible without its authors. Whom to thank (or shake a fist at?) for each puzzle?

Doug Lloyd, for each of the odd (ha!) puzzles, including #1 Logic Gate, #3 Word Ladder+, #5 Hydrocross, and #7 Medal Table
Matthew Warshauer, for the others, including #2 Mastermind, #4 Nonthreatening, #6 Rolling the Dice, and #8 To Completion.

Speaking of #7, the memes online were strong this year:

Perhaps our favorite comment, though, among all the posts online:

It was really tough but also interesting..
A new friend of mine help me..
Thanks to her..(First time felt that the other countries people are so cooperative..she changed my thinking about british..)

And, now, the solutions you’ve been waiting for, thanks to CS50’s own Doug Lloyd, Lauren Scully, Ian Sexton, and Ramon Galvan! Spoilers ahead!

tl;dr, the correct answers were: