This was a very popular problem with lots of solutions sent in. An impressive performance from many of you. The choice of names to list and solutions to show was a difficult one but I have chosen ones which give a spread of approaches and demonstrate that there is more than one way to crack a nut!

Tiffany of Island School offered this solution, similar to those sent in by many of you:

Daniel from Wales High School offered the following. I liked this because he made an effort to explain every step:

Haukur of King's School Canterbury saw a pattern in the mathematics and utilised it. Not the most concise solution but a lovely example of using pattern as a key to problem solving.