What’s the Point? Calculations with Decimals
If you’ve been following activity in my free Facebook community this week, you’ll know I’ve been giving some pointers on calculations with decimals. Our children need to understand why decimals exist and how to calculate with them and be able to convert between fractions, decimals, and percentages. Having this understanding helps them to have fluency with numbers, which is just like having fluency in a language, they stop stuttering when doing other maths.
We work in base 10, which means we move to the next column after going up 10 digits.
Did you know that the Babylonians worked in base 60? That’s why time is in 60s, and there are 360 degrees in a circle. I think working in 10s makes much more sense especially as we have 10 fingers.
Dec comes from the Greek word for 10 déka, and deci means a tenth. Having decimals allows us to split numbers up into equal parts of 10, which is really useful!
Multiplying by powers of 10
Move the numbers not the decimal point. When multiplying by powers of 10 (10, 100, 1000…) it can be tempting to move the decimal point. The problem with that is it doesn’t help with the understanding of numbers and place value. It’s much better to keep the decimal place static and move the numbers!
To help with this you can use place value charts. Maths Salamanders have some printables that are really handy.
Doing Calculations with Decimals Adding and Subtraction
I recommend adding and subtracting using a column method, so that you can line up the decimal point. If there are gaps you can fill them with zeros.
Multiplying with decimals
For this I find it’s best to ‘remove’ the decimal points by multiplying by powers of 10. Do the calculation and then put the decimal point ‘back’ by dividing by the same powers of 10.
Dividing with decimals
Although this isn’t any trickier than the others it requires an understanding that division is the same as a fraction, or is a ratio. That way you can multiply both numbers by a power of 10 to have a calculation that will give the same answer.
If your child doesn’t have that understanding of division they will then do the same as multiplication and divide by the power of 10, but they don’t need to do that!
Fractions, Decimals and Percentages
Converting between these is a useful skill, especially as using the decimal equivalent of a percentage is needed for percentage calculations. It’s important that your child understands that they are the same thing, just different ways of representing that number.
I go into all of these calculations with decimals and more, in more detail, when practicing modern maths methods in a course for parents called Foundations4Homework. Make sure you don’t miss out by joining my free Facebook group Education Support 4 Parents.