Estimation in Subtraction
Estimating in subtraction means finding an approximate answer quickly — without working out the exact value. It is a vital checking tool.
Why Estimate?
- To check whether a calculator or worked answer is reasonable.
- To make quick decisions (e.g., can I afford this?).
- To spot errors before they become problems.
Step 1 – Round Each Number
Round each number to a convenient place value (nearest ten, hundred, etc.), then subtract the rounded values.
Worked Examples
| Calculation | Rounded | Estimate | Exact |
|---|---|---|---|
| 87 − 32 | 90 − 30 | 60 | 55 |
| 548 − 213 | 500 − 200 | 300 | 335 |
| 4,762 − 1,895 | 5,000 − 2,000 | 3,000 | 2,867 |
Front-End Estimation
Use only the leading (front) digits for a quick rough answer.
7,489 − 3,145 → 7,000 − 3,000 = 4,000 (exact: 4,344)
Decimal Estimation
8.73 − 3.21 → 9 − 3 = 6 (exact: 5.52)
Key Takeaways
- Round before you subtract.
- The estimate should be close — if it isn't, check your rounding.
- Estimation is not the same as the exact answer; it is a sanity check.
- Over-estimating or under-estimating depends on whether you rounded up or down.
Practice Questions
- Estimate 76 − 43 by rounding to the nearest ten.
- Estimate 932 − 487 by rounding to the nearest hundred.
- Estimate 6,155 − 2,840 using front-end estimation.
- A jacket costs £87.50 and a shirt costs £23.99. Estimate the difference in price.
- Estimate 0.95 − 0.38 by rounding to the nearest whole number.