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The Kid Should See This

Can you solve the birthday cake riddle?

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“Your friend’s birthday is tomorrow, and he’s turning… well… you’ve forgotten. A ginormous cake has been prepared and your job is to sculpt his age as the chocolate centerpiece. The birthday boy is a giant, and you’re afraid that if your forgetfulness becomes known, you’ll become part of the feast.”

How can you figure out your friend’s age before the party? Pause this birthday cake riddle TED-Ed video by Marie Brodsky, with animation by Igor Coric and Artrake Studio, when you see the instructions. They’re also shared in the image below. To learn how to figure out this type of logic puzzle, continue watching the second half of the lesson.

giant friend
birthday cake riddle rules
Then be sure to Dig Deeper with TED-Ed:

“Fun fact: the classic version of this riddle casts you as a train conductor trying to count the number of train cars in a train, where all you can do is walk from one car to the next and turn the light on or off! Here is the puzzle as described by the riddle section of FiveThirtyEight. If you want to dig deeper into the math, Professor Tim Black has a nice blog about this puzzle analyzing the efficiency of different solutions and Ben Tupper has a video walking through several possible approaches.”

A bit slower and quieter, this video is very helpful for understanding the check(n) algorithm in a methodical way:

Ben Tupper Train Car video

“Puzzles like this one challenge your brain to figure out something new, with no clear ties to problems you already know how to solve! The more of these you tackle, the more equipped you become at adapting to new challenges that come your way. Think back to how impossible this problem seemed at first, and reflect on the ‘aha’ feeling you may have experienced once you realized returning to the starting candle would help.”

talking with the baker
Watch more symmetry, algorithm, logic, and math videos:
• What’s the fastest way to alphabetize your bookshelf?
• Why are algorithms called algorithms?
• Symmetry, an Eames animated short for the 1961 Mathematica exhibition
• Russian Multiplication, an astonishing way to multiply
• The Scientific Way to Cut a Cake
• How to equally share a cake between three people

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