This is a PDF of the slides I used in a lesson on function machines. I started the lesson with a recap of adding and subtracting directed numbers.
Quality AssuredCategory:MathematicsPublisher:Department for Education
This is a very customisable interactive resource for teaching function machines. It can do one or two step machines.
This is one of the CIMT resources. This link is the interactive version, so the children get immediate feedback on their answers
This is the .pdf version of the CIMT resource on function machines.
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I used this at the end of a lesson on function machines so that I could check what the students had learned.