### Odd Differences

The diagram illustrates the formula: 1 + 3 + 5 + ... + (2n - 1) = n² Use the diagram to show that any odd number is the difference of two squares.

### Factorial

How many zeros are there at the end of the number which is the product of first hundred positive integers?

### Rachel's Problem

Is it true that $99^n$ has 2n digits and $999^n$ has 3n digits? Investigate!

# Really Mr. Bond

##### Age 14 to 16 Challenge Level:

Is it always the case that when you square a number whose last digit is 5 you always end with 25?

By breaking the number down into a form (x + 5) it may then be possible to see what is happening and why.