What is the greatest common factor of 1218 and 42?

Answer 1

First, using your 2 divisibility rule, we find that both are even, therefore both are divisible by 2. Now, using the 3 divisibility rule, we know that 1 + 2 + 8 + 1 = 12, and 4 + 2 = 6, both multiples of 3. Thus, it must be divisible by 3 and 2, and it must divisible by 6! If so, then dividing both by 6 yields:

1218/6 = 203, 42/6 = 7.

Now both values are prime, therefore, 6 is the GCF of 1218 and 42.