Teacher notes

• Generally, when doing mental calculations, students should be encouraged to begin with one number and use place value partitioning for subsequent numbers, since this requires students to ‘hold’ just one number in their head.
• Students may need to be reminded that beginning with the largest number makes the calculation easier. Students will need to understand the commutative property.
• It is possible to find the answer by partitioning both numbers. However, partitioning both numbers should be discouraged as students may get confused when using this method for subtraction which requires ‘breaking a ten’.
• Using place value materials to represent the addends and insisting on appropriate language (for example, “I will add two tens to these three tens to make five tens” or “I will add 20 to 30 and make 50”) are helpful ways of promoting and making explicit students’ understanding of the quantities involved. Students will sometimes abbreviate their explanations (for example, “I will add the 2 to the 3 and make 5”). These abbreviations should be discouraged as referring to the digit rather than its value affects a student’s ability to recognise when their answer does not make sense.
• Students who are very familiar with facts that make ten find it easier to partition numbers in such a way that they roll over the ten. Using place value materials such as tens frames, number lines or hundreds charts assists students who do not have this knowledge to identify the fact they need.