Multiply Decimals by Rounding to Whole Numbers

In this activity, we'll be looking at how we can multiply decimals without using a calculator to estimate the answer.

But, how do we do this?
 

First, we round both decimals to the nearest whole numbers, and then we simply do the multiplication calculation.

Example

4.11 × 8.75 = ?

Here, we round 4.11 down to the whole number 4, and we round 8.75 up to the whole number 9.

(Remember, if the number after the decimal point is 5 or higher we round up, if it's a 4 or lower then we round down.)

So, now we have the following calculation, which we can work out using our times table knowledge.

4 × 9 = 36

Easy, right?! 

Let's try some questions.