Editor’s note: This is an updated version of a blog post published on April 29, 2017.
If you’ve ever said ‘just add a zero’ when teaching how to multiply by 10, your learners could be missing out. Avoid this common maths misconception and give learners a deeper understanding of the base-ten system.
One particularly prevalent maths misconception that often gets taught is adding zeros when multiplying by a power of 10. Teachers don’t deliberately set out to teach this misconception but many have inherited it from when they were in school, innocently reproducing this flawed maths in their own practice.
“Just Add A Zero”
While being taught, 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 as some sort of ‘recipe’ instruction but ‘adding a zero’ is like cooking the books, it’s maths fraud!
Blind acceptance of 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.
The ‘adding a zero’ trick is commonly taught as a helpful rule when multiplying by 10 but it prevents learners from developing a deeper understanding of the base-ten system.
Imagine ‘adding the 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 elsewhere such as 00.05 because they are unfamiliar with decimals.