Solving number sentences successfully using relational thinking certainly calls on a deep understanding of equivalence. Students need to know the direction in which compensation has to be carried out in order to maintain equivalence (Kieran, 1981; Irwin & Britt, 2005; Stephens, 2006). Some children who correctly transform number sentences involving addition reason incorrectly that a number sentence such as 87 – 48 can be transformed to be equivalent to 90 – 45. These children do not understand the direction in which compensation must take place when using subtraction or difference. They fail to recognise that the relationship of difference is fundamentally different from addition. Other children, however, recognise this feature explaining that in order for the difference to remain the same, the same number has to be added to (or subtracted from) each number to the left of the equal sign. These children write correctly 87 – 48 = 89 – 50. The first part of this study probed children’s thinking with number sentences.