So,
To find the GCF, you have to factor all of the numbers.
21 --> 3 * 7
24 --> 2 * 2 * 2 * 3
27 --> 3 * 3 * 3
Next, you find all the factors that they have in common.
They all have at least one 3, so that means that 3 is the greatest common factor of 21, 24, and 27.
