Express 225 as the sum of odd numbers.

We know that $n^2$ can be written as sum of first $n$ odd number.
We can write 225 as ${15}^2$.
(1 mark)

So, for expressing 225 as sum of odd numbers we can express it as sum of first 15 odd numbers.
(0.5 marks)

225 = 1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 17 + 19 + 21 + 23 + 25 + 27 + 29.
(1 mark)