Oliver, Rushad and Rayan from Tanglin Trust School in Singapore, and Jessica from Maundene School all sent clear solutions. The pupils from Tanglin Trust School wrote:

First we joined the dots and it formed a square.
We did $10$ times $10$ then divided by $2$ to get the area of the square, since the excess space is the same amount (area) as the square. So the area must be $50$cm squared.

That's right - well done. The second part of the question was slightly more difficult, but Jessica arrived at the solution - $49$ tiles. Can you see why this is correct?