Although Matilda was supposed to be 5, I'd challenge any 5 year old to actually do this maths in their heads! If you are reading Matilda to a class, this is a very nice way to introduce Spreadsheets in KS2 or could be used as an exercise in KS3. You will need the book Matilda by Roald Dahl (Pages 45 - 47) to make full use of this exercise, but could be done as a standalone. Also a good example of real life Maths for a Maths lesson. (I would stress this is based on the book rather than the film, which has different Maths)