Presenter notes: Open with two marks on the board: 17 out of 20 and 41 out of 50. Ask which is better and take a vote. Many will pick 41 because it is the larger number, which is exactly the difficulty percentages exist to solve. Then ask why every shop sign, every exam result and every news report about elections uses percentages rather than raw counts. By the end of the lesson they will be able to answer that in one sentence.