In this paper, we present a study on how pre-service primary school teachers solve simple
addition problems involving negative number. We analyse the strategies and reasoning they
follow in solving questions in this topic considering the structure of the problems (change,
compare, change–compare and equalize) and the position of the unknown. The results reveal
gaps in the numerical knowledge of those pre-service primary school teachers analysed,
gaps that are filled in by their pre-existing knowledge of positive numbers.