Try doing the division, in this case, 81 / 3. If you get a whole number, it is; otherwise it isn't.
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The Greatest Common Divisor of two numbers is the largest number that will divide into both evenly. For example, although 27 and 81 are both divisible by 3, it wouldn't be the greatest common divisor, because 27 divides into 81 evenly as well: the greatest common divisor of 27 and 81 is 27.
The Greatest Common Divisor is 3
9 * 9 = 81 and3*3*3*3 = 81
81 is not a prime number. 81 = 3 * 3 * 3 * 3
greatest common divisor of 60 and 231 is 3.