Estimate Differences by Rounding

Estimating a Difference by Rounding

When you estimate a difference, you replace the original numbers with nearby rounded numbers that are easier to subtract. This gives an answer that is close to the exact result and quick to find.

1) Round each number

Choose a place value that makes the subtraction simple, such as the nearest ten, hundred, or thousand. Round each number separately.

2) Subtract the rounded numbers

Use the rounded values to compute the difference.

3) Compare with the exact difference

If possible, check your estimate by doing the original subtraction. The estimate should be reasonably close, though not necessarily exact.

Example idea

If a problem asks for an estimate of 487 − 193, you might round to 500 − 200 = 300. That is the estimated difference.

Quick check

• Did you round both numbers the same way?
• Is the estimate close to the exact answer?
• Does the result make sense for the size of the original numbers?

Use this method when you want a fast, reasonable answer before or instead of an exact subtraction.