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 (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. These students fail to recognise
that the relationship of difference is fundamentally different to 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 each number in the expression. These children write correctly 87 – 48 = 89 – 50. The
first part of this study was designed to probe children’s thinking with number sentences