Why is this Wrong? Decimal Addition and Subtraction

When marking maths, it can be very frustrating when pupils get the answer wrong when we know that they understand how to carry out a particular calculation. This can often be due to a careless error that a pupil has overlooked since they are not asking themselves, ‘Is this answer reasonable?’


For this reason, it is useful to look at questions and answers and ask pupils to explain why particular answers cannot be correct, without carrying out the actual calculation. That is the focus of this PowerPoint presentation.


It is worth removing any writing materials, including whiteboards, so that pupils cannot work out the actual calculation but have to rely on reasoning and estimates they can complete in their heads.

Why is this Wrong? Decimal Addition and Subtraction

    • 35 interactive slides each containing 4 cards that are flipped to reveal a calculation. An incorrect answer is provided for each calculation.
    • Printables and suggested reasons for how we know the answers are wrong are included to minimise preparation time and promote discussion.

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