Mathematical writing is one way for primary students to communicate their mathematical thinking. Research in the field of writing has shown that to become an effective teacher of writing, preservice teachers must have experience engaging in the kinds of writing given to their students. The study reported in this paper explored how 27 preservice teachers enrolled in an elementary mathematics methods course explained their reasons for selecting strategies to solve equations that required making either addition or subtraction calculations. Element scoring was used to analyse the preservice teachers' reasons for selecting the strategy and the composition of their mathematical writing. Results indicated that the majority of preservice teachers included a mathematical reason for selecting their strategy, however; the reasons varied in their connection to the mathematics discipline. This study contributes to the existing literature on mathematical writing by providing insight into how preservice teachers explain their mathematical reasons for selecting a computation strategy; thereby, demonstrating the need for increased opportunities for preservice teachers to write.