We can estimate answers to complicated additions by rounding both decimals to the nearest whole number before adding them together.
How to round decimals to whole numbers
Look at the number directly to the right of the decimal point, that is the number in the tenths position.
If that number is less than 5, we round down so that we simply remove the numbers after the decimal point.
For example: 32.46 rounds down to 32 because the number in the tenths place is a 4.
159.08 rounds down to 159 because there is a zero in the tenths place.
If the number in the tenths place is 5 or more, we round up.
For example: 72.6 rounds up to 73 because the number in the tenths place is a 6.
197.57 rounds up to 198 because the number in the tenths place is a 5.
Having seen how to round decimals to the nearest whole number, let's look at how to use this to estimate addition of decimals.
64.51 + 18.15 can be estimated by rounding both decimals to the nearest whole number first: 64.51 rounds up to 65 and 18.15 rounds down to 18.
65 + 18 = 83
So the answer is 83.
Let's have a go at some questions now.