Famous Math Riddles That Went Viral on Reddit
Reddit isn’t just a hub for memes and debates — it's also home to some of the most mind-bending math riddles that have stumped millions. These puzzles often start in niche math or brainteaser subreddits and quickly spread across the internet, sparking debates, detailed breakdowns, and thousands of comments.
Here’s a roundup of some of the most famous viral math riddles from Reddit — and what made them so unforgettable.
1. The “1 = 2” Algebra Trick
This one feels like math betrayal. The riddle presents a seemingly valid algebraic proof that concludes with 1 = 2. Here's a simplified version:
Let’s say:
a = b
Multiply both sides: a² = ab
Subtract b²: a² − b² = ab − b²
Factor: (a + b)(a − b) = b(a − b)
Cancel (a − b): a + b = b
Since a = b, replace: b + b = b → 2b = b → 2 = 1
The catch? You can’t divide by zero — and (a − b) is zero. That’s the trap. Reddit loved it because it’s both clever and wrong in just the right way.
2. The Viral “Order of Operations” Debate
You’ve probably seen this one:
6 ÷ 2(1 + 2) = ?
This riddle lit up Reddit with people split between two answers: 1 and 9.
Here's the breakdown:
- Step 1: Parentheses → 6 ÷ 2(3)
- Now here's the debate: Is it
- 6 ÷ 2 × 3 = 3 × 3 = 9?
- Or 6 ÷ (2 × 3) = 6 ÷ 6 = 1?
Most math teachers (and the official order of operations) say it’s 9, but the confusion comes from how the expression is written. This debate taught everyone one thing: write your equations clearly.
3. Cheryl’s Birthday Puzzle
Originally a Singapore math contest question, this riddle made its way to Reddit where it blew up.
Cheryl tells two friends, Albert and Bernard, her birthday is one of the following:
May 15, May 16, May 19
June 17, June 18
July 14, July 16
August 14, August 15, August 17
She tells Albert the month, and Bernard the day. Based on a short conversation between them, you're supposed to figure out Cheryl's birthday.
Reddit loved trying to unravel the logic — and the challenge became a viral sensation. The answer (spoiler: July 16) required careful elimination based on the friends’ statements.
4. The “100 Prisoners and a Light Bulb” Problem
This riddle is more of a strategy puzzle, but it took off big time:
There are 100 prisoners. They’re isolated but occasionally, one is brought into a room with a light bulb. They can toggle it on/off. At any time, one prisoner can declare: “All 100 prisoners have been in this room.” If they're right, everyone is freed. If they’re wrong, everyone is executed.
They must create a strategy ahead of time. The Reddit thread exploring all the possible methods was epic — showcasing not just math, but logic, memory, and teamwork.
5. The Monty Hall Problem
This classic probability problem found new life on Reddit, especially when framed as a riddle.
You’re on a game show. Three doors. Behind one is a car; behind the other two, goats. You pick a door. The host, who knows what’s behind the doors, opens one of the other doors to reveal a goat. You’re asked: do you want to switch?
Most people say it doesn’t matter — 50/50, right?
Wrong. If you switch, you win 2/3 of the time. Stay, and it’s only 1/3.
Reddit threads explaining this exploded, with simulations, arguments, and eventually — understanding.
6. The Missing Dollar Riddle
Three people check into a hotel room costing 10. Later, the manager realizes the room was only 5 to return.
The bellboy, sneaky, keeps 1 back to each guest. Now each person paid 27. Add the 29.
Where’s the missing dollar?
It’s a logic trap. The math doesn't actually work that way — you’re mixing totals in the wrong direction. But Reddit users couldn’t resist debating it for days.
7. The Camel Banana Problem
This one’s a brain burner from r/math:
You have 3000 bananas and a camel that can carry only 1000 at a time. For every mile walked, the camel eats one banana. You need to transport as many bananas as possible across 1000 miles of desert.
How many bananas can you get across?
The answer isn't zero — and figuring out how to “stage” bananas along the way sparked wild strategies. Redditors approached it like a real-world logistics challenge.
Final Thoughts
Math riddles that go viral often do so because they’re just tricky enough to feel unsolvable, but simple enough to make you go "Ohhh!" once you crack them.
They’re fun, frustrating, and incredibly satisfying — and thanks to Reddit, they’ve become part of internet culture.
Try a few with your friends or students. Just be prepared for a very lively debate.