We can estimate answers to complicated subtractions by rounding both decimals to the nearest whole number before subtracting them.
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 subtraction of decimals.
We can estimate the answers by first rounding both decimals to the nearest whole number and then finding the difference between them.
45.7 - 16.4 can be estimated by rounding each number to the nearest whole number.
45.7 is rounded up to 46 and 16.4 is rounded down to 16
46 - 16 = 30