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From A) 2531 is a prime so M=q* 2531. Since 2531 is the GREATEST common factor, or G, then G-2531 so A SUF

How do you quickly determine if a number is prime?

Does anyone have any answer to this? I wouldn't be able to tell just by looking at the problem that 2531 is a prime number. Any shortcuts to calculate this?

Guys please remember me the rule when the digit is divided by 3, when by 4, by 6, by 7, by 8, by 9.
I forgot it. Just remember something like when the sum of the last 2 digits of any number is divided by 4 the number is divided by 4.