How to do Decimal Mental Math

Tips and Tricks

Decimal mental math is a really simple thing that only you can  perform because it has to deal with how you do math in your head. In order to do decimal mental math, all you need to do is follow some of these super easy steps!

The main thing that you need to focus on when doing decimal mental math is that you have to separate the whole numbers and the decimal numbers. I call this the Separating-Method and what i mean by that in a decimal number the numbers before the decimal point are whole numbers and the numbers after the decimal point are decimal numbers. After doing that, all you do is apply the operation. What I mean by that is that if your operation is addition, then you'll add. If it's subtraction, therefore, you'll subtract. Simple, right?! Now that you have learned and mastered a way to do mental math let's move on. I'll give you an example, and then provide you with some practice problems. 

Examples

Solve 3.4+6.9

Remember, first we must do the Separating-Method. So, that means that we will end up with 3+6 and 4+9. Now, remove the addition signs and make that 36+49. Then add that in your brain. If you are struggling, then use the commutative properties and the associative properties of addition. These make it easier to do mental math. First, separate the compatible numbers, which are 30+40, which equals 70. Then do 6+9, which equals 13. 70+13, therefore equals 83. Now, time to do the final step- adding the decimal point in your answer. If you look back at the original problem and place the answer using place value, you should come up with 8.3. So, the answer to 3.4+6.9 equals 8.3!

I know it looks like a lot of mental math, but trust me, it gets so much easier once you've practiced enough. So, try some of these problems out!

 Sheet 1 Practice Problems Decimal Mental Math