Challenge Level

Gabriel's Problem and Counting Factors might be good problems to try before looking at this one.

If you know the prime factors of a number, how can you work out which two-digit numbers are factors?

It might help to draw a factor tree.

If you'd like some help with the **extension**, click below

What is the largest that the

What is the smallest that the

What does this tell you about the

Now it might be helpful to think about the units digits.