This study examines if interdisciplinary teaching can be said to facilitate the learning and use of fractions by Swedish 12-year-old pupils. Home and Consumer Studies is well suited to interdisciplinary teaching, and young people can therefore find it interesting to study maths since the setting is relevant to them. Building on variation theory and a learning study, we examined pupils’ (n18) ability to double fractions greater than ½ when using a recipe. The general results show that what is to be learned benefits if it is presented in different ways, that teachers should not take pupils’ knowledge for granted, and mathematically that it is not necessary to divide something to be able to double it. We argue that the study shows that genuine problems based on pupils’ interest and life world can enhance motivation and, in turn, learning.