Should you just add a zero?
Learners often hear this misconception: “when multiplying by 10 just add a zero”.
This approach is limited in its usefulness and can be damaging. It’s almost taught like a sort of recipe but ‘adding a zero’ is like cooking the books, it’s maths fraud!
Blindly accepting this method for multiplying by 10 means pupils apply this rule incorrectly because they don’t appreciate the underlying mathematics at work. The ‘adding zeros’ trick can work when multiplying whole numbers by powers of 10, for example, 678 x 10 = 6780, 213 x 100 = 21300, 34 x 1000 = 34000, but this method completely falls down and is totally unsuitable when multiplying a decimal value by a power of 10.
In this example, 9.5 x 10 isn’t 9.50 because simply inserting a zero on the end gives exactly the same value.
Try this one with your class:
10 x 10 =
10.0 x 10 =
It’s possible that you’ll get 100 and 10.00 as responses.
Although taught as a helpful rule when multiplying by 10, ‘adding a zero’ is a maths misconception that stops learners from developing a deeper understanding of the base-ten system.
Imagine ‘adding a zero’ when multiplying 0.05 by 10. Pupils who have learned this rule might answer 0.050, or they could decide to add the zero somewhere else because they are unfamiliar with decimals. I’ve seen learners come up with 00.05.