Multiplying decimals: problems with tricks
ExampleTo calculate \(0.3 \times 0.2\), start by performing the simple calculation \(3 \times 2 = 6\).Then count the number of decimal places in the question, which is two: one in 0.2 and one in 0.3, so the answer must also have two decimal places hence the answer is 0.06.In this case, the trick works. But if we apply the trick to \(0.4 \times 0.15\) the trick does not appear to work. The question has three decimal places but the answer only has two decimal places. In the video, Michael explains why.
Teaching resourcesIf you enjoy deconstructing calculation ‘tricks’ in mathematics, you might want to read ‘Nix the Tricks’ by Tina Cordone. The book looks at tricks and short cuts used in maths, explains why they are so damaging, and then provides an alternative method that teaches for understanding.
Problem worksheetComplete questions 1 and 2 from this week’s worksheet.
As a reminder, the worksheet is found in the first unit of this week.
Maths Subject Knowledge: Fractions, Decimals, and Percentages
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