#### You may also like Make a set of numbers that use all the digits from 1 to 9, once and once only. Add them up. The result is divisible by 9. Add each of the digits in the new number. What is their sum? Now try some other possibilities for yourself! ### N000ughty Thoughts

How many noughts are at the end of these giant numbers? ### DOTS Division

Take any pair of two digit numbers x=ab and y=cd where, without loss of generality, ab > cd . Form two 4 digit numbers r=abcd and s=cdab and calculate: {r^2 - s^2} /{x^2 - y^2}.

# Funny Factorisation

##### Age 11 to 16Challenge 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 four-digit number can be?
What is the smallest that the two-digit number can be?
What does this tell you about the three-digit number?