31. Determine whether each of the following numbers is prime or composite.
(a) 231 (b) 393

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  1. Try the divisibility rules:
    A number is divisible by 2 if the last digit is even.
    (a) ..1 NO
    (b) ..3 NO

    A number is divisible by 3 if the sum of the digits is divisible by three.
    (a) 2+3+1=6/3=2 with no remainder
    (b) 3+9+3=12/3=4 with no remainder

    Can you make the conclusions?

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