Ten as a unit (Level 1)
Demonstration of place value understanding using ten as an abstract unit is in limited situations at Level 1, typically where one of the two-digit numbers in a sum is taken as a represented quantity.
It is also important to realise that subtraction tasks involving trading (i.e. where the digit in the units place in the minuend is greater than the digit in the units place in the subtrahend) make greater demands on conceptual place value than additions. The following response shows a classic error in subtraction, sometimes described as ‘the smaller from larger subtraction bug’, when there is not a conceptual understanding of multi-unit place value.
The student subtracts the tens and ones separately but subtracts 3 from 7. The smaller from larger subtraction bug occurs when a student always finds the difference between the values of the digits irrespective of the order of subtraction.