This paper looks at how teachers can support students' additive thinking. It reports on a study of a lesson designed to teach the equal additions strategy for subtraction, contained in Numeracy Development Project (NDP) Book 5. Nine teachers were observed teaching mathematics. Despite having a strong commitment to promoting conceptual understanding, many teachers struggled with various aspects of the material and resorted to teaching procedurally. We concluded that teachers need to have a deeper, more connected understanding of addition and subtraction in order to develop their pedagogical content knowledge in mathematics.