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A prime only has itself and 1 as factors.

If you can divide another number besides 1 and itself, then you know the number isn't prime.

A way to do a prime test by hand is to test if the number is divisible by any prime numbers between 1 and the number's square root.

For example, if you want to see if 337 is a Prime number, you first take its square root (= 18.36). Now check if 337 is divisible by any prime number between 1 and 19: 2, 3, 5, 7, 11, 13, 17. You will see none of them are possible without fractions, therefore 337 is a prime number.

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Q: How do you check whether a number is prime or not?
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