> Why is the math a nonstop repeating decimal when existence allows 3 pieces?
It’s a representation problem because you are using [base 10](https://en.wikipedia.org/wiki/Decimal). Like the other poster said, imagine every marble can be split into 10 sub-marbles, and those submarbles can be split up into 10 sub-sub marbles, and so-on.
So if you split up into **any multiple of 2 or 5**, it will be exact. (For example, if I have one marble, I can split into 10 sub-marbles. Now I can divide them by 5 or 2, or by 10.) That means splitting up into 2, 4, 5, or 8 is easy.
On the other hand, splitting thing up into **3, 6, 7 or 9** pieces requires dividing by 3 or 7, which is not possible when all you have are fractions of 10. Ditto for any larger numbers not evenly divisible by 2 and 5. For example, if you look at all the numbers between 20 and 30, only 20, 25 and 30 are OK. The rest will repeat. (NOTE: Numbers like 1/22 or 1/3 are still [rational](https://en.wikipedia.org/wiki/Rational_number) numbers: They can repeat, but only in a repeated pattern, unlike irrational numbers which have no pattern.)
Computers have the same problem, but worse: They use binary, which is base 2. Anything that is not divisible by 2 is a problem. That means “1/10” [can not be represented in floating point](https://www.exploringbinary.com/why-0-point-1-does-not-exist-in-floating-point/) — it requires an infinite decimal.
> Is the assumption that it’s physically impossible to cut something into 3 perfectly even pieces?
Well, it depends. Let’s say you have a pie, and you are trying to divide it up into 3 equal pieces. How would you know if they are equal? The truth is, the only way you get it 100% evenly divided is if the number of atoms/molecules in the pie was **exactly** a multiple of 3. If there were an even number of Atoms, it is impossible to slice the pie into 3 equal parts.
For the same reason you just eyeball the pie and say “those 3 parts look are equal”, you shouldn’t worry about the accuracy of truncating a repeating decimal. For example, NASA only needs [15 decimal places of PI](https://kottke.org/16/03/how-many-digits-of-pi-does-nasa-use) to send out probes to the entire solar system.
Define the item as having length 3. Then you can partition it into three pieces of length 1.
Define the item as having length 1. Then you partition it into three pieces of length 1/3=0.333…
They’re all real numbers. The last one is a real number too. Don’t get hung that it’s a repeating decimal. That’s a consequence of base 10 here.
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