Download the lesson here.
Completely revamped lesson. Includes an example problem pair, lots of mini-whiteboard work, a regular activity (which is still kind of rubbish to be honest. I could do with thinking about these questions more) and a nice little codebreaker.
I prefer setting my code breakers out like this. I find if you do ‘collect a joke’ kids will just try and guess the joke. The answer here (au4a83 ) is how you type ‘password’ on a Taiwanese keyboard. From this very interesting article (not really maths related).
Also added some exam questions.