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# Can you solve the risky disk riddle?

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Malicious code has hijacked your mainframe, but luckily you’ve seen this before: The malicious code specifically “corrupts one of the 4 disks that run your mainframe, represented by lights showing which are on and which off.”

If your antivirus squad can destroy the corrupted disk, the malicious code will be defeated, but if you choose wrong, the entire system will be wiped out. How can you tell which of the four disks has been corrupted? Can you solve the risky disk riddle?

This TED-Ed lesson by mathematician James Tanton, with animation by Igor Coric of Artrake Studio, shares the thinking behind solving this challenge. Pause the video at 1m20s if you want to figure it out on your own.

“The solutions to these puzzles make use of the binary representation of numbers. For a fun, straightforward, and visual way to understand the binary number system have a look at the opening explorations of the Exploding Dots story from the Global Math Project. (Millions of students and teachers have!)

“Solving the puzzle for a count of discs a large power of two relies on counting the 1s that appear in the binary representations of numbers and keeping track on whether these counts are even or odd. Mathematicians use the term parity to describe the idea that an object in a certain scenario might be in one of two states: up or down, black or white, or, in the case of the counting numbers, even or odd, for instance.”

How does binary code work? Watch that video next. Then check out this Binary Marble Adding Machine.

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This video was posted 5 months ago.