"I don't need a guide!" objected the Doctor, tapping two more squares to create another glowing bar as soon as the face had finished speaking. He was frowning with concentration, but he was still in his element.

To Billy, he added: "It could be a few different things. If it's algebra, it's 1.3. Or it could mean that we're supposed to take three and four as equal. Which can then be used to infer that five and six are equal. Or the first bit is just a red herring and we should just say what four times five is."