There is no quick way to work out if a number is prime or not, the only way is to see if it can be divided by any numbers that aren't itself or one.
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Work out if they have any factors, other than the number 1 and themselves. If they do not they are prime.
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It is 97. it is largest, because it is the largest double digit prime number.
But you do know the factors of prime numbers. Every prime number has two factors: one and the number itself.
Memorize them all.