Estimate a sum by rounding it to the greatest place value by completing three steps. First, round each value in the equation to the greatest place value. Second, add together the numbers in the greatest place values by reducing the numbers to single digits. Third, convert the single digits back into the original place values.
In order to round each value to the greatest place value, look at the second digit in the number. If it is 5 or higher, then the first number becomes one greater, and the rest of the digits become zeros. If it is 4 or lower, then the first number stays the same, and the rest of the numbers become zeros. This leaves you with two numbers that are easy to add together, since they are only single-digit approximations of the original set of numbers, making it easy to find the sum of the numbers.
For example, when finding the sum of 2,385 + 6,825, begin by rounding both numbers to the greatest place value. This leaves 2,000 + 7,000. Second, add together the two single digits: 7 + 2 = 9. Finally, convert this single digit back into the thousandths place that you had rounded it to, leaving you with an estimate of 9,000.