### 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}.

### Mod 3

Prove that if a^2+b^2 is a multiple of 3 then both a and b are multiples of 3.

# Common Divisor

##### Age 14 to 16Challenge Level

Find the largest integer which divides every member of the following sequence:

$$1^5-1,\ 2^5-2,\ 3^5-3,\cdots\ n^5-n.$$