The factors of a number are <em>all of the positive whole numbers that go into it evenly</em>. For example, 2 is a factor of 6 because 6 ÷ 2 = 3, or equivalently, 6 = 2 x 3. Note that this naturally implies that every whole number has 1 and itself as a factor.
The numbers that go evenly into 24 are 1, 2, 3, 4, 6, 8, 12, and 24. For 27, those numbers are 1, 3, 9, and 27.
The <em>common factors </em>between two numbers are numbers that go into <em>both </em>numbers evenly. Looking through the lists for 24 and 27, we see that 1 and 3 are their common factors. The <em>greatest</em> common factor is, of course, the largest of these, so the greatest common factor of 24 and 27 is 3.
