See whether the number is divisible by 2, and by all odd numbers up to the square root of the number. For numbers up to 100, it is enough to check the factors 2, 3, 5, and 7. For higher numbers you need to check more factors.
For very large numbers (for example, hundreds of digits), more efficient methods are known, but those methods are also more complicated.
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Find a prime number, add 2 to the number. Check if the new number is prime. IE : 3 is prime. 3+2 =5. 5 is prime. (3,5) are twin primes.
29 is a prime number, meaning it has no smaller factors. For any number up to 120, to check whether it is prime or not, it is sufficient to check whether it is divisible by the first four prime numbers (2, 3, 5 and 7).
You just have to work out it,take each number below it and check whether it is prime or not.
It is 29 because 29*23 = 667
You can write out this algorithm. This will then be programmed into the device to make determining prime numbers easier.