Round numbers with the decimals up to the next whole number if the number after the decimal is 5 through 9. For example, if you have 3.6, you would round the figure to 4.
Round numbers with decimals down to the nearest whole number if the digit after the decimal is less than 5. For example, you would round 7.2 to 7.
Add the rounded figures to estimate the sum. For instance, consider the sum of the following numbers: 6.8, 3.1 and 8.7. You would round 6.8 to 7, 3.1 to 3 and 8.7 to 9. Then you would add 7, 3 and 9 to get an estimated sum of 19.
Subtract one rounded figure from another to find an estimated difference. For example, consider subtracting 10.23 from 20.57. You would first round 10.23 to 10 and 20.57 to 21. Then you would subtract 10 from 21 to get an estimated difference of 11.