This study focuses on teachers' engagement with tasks based on the income tax tables issued by the South African tax authorities. The participants in the study are a group of 37 teachers who were enrolled in an in-service programme for Mathematical Literacy teachers. The purpose of the study is to explore the teachers' interpretation and use of the rule used to calculate income tax. Data were generated from written responses of the teachers to three tasks, as well as follow-up interviews with eight of the participants. The findings indicate that some teachers (8%) did not recognise any of the demands inherent in the income tax rule that they teach to their learners. Most teachers (54%) were in the novice category, showing that they met some of the demands but need some help in carrying out the rule fluently. A further 32% were able to use the rule to work out the tax given various input incomes, but could not use the rule to find the input income when given the tax output, because they did not have the necessary algebraic skill.