Addition & Subtraction • Topic 5 of 5

Estimation

Estimation is the skill of getting close on purpose, a reasoned approximation that is good enough for the job, and NCF treats it as a core process, not an afterthought. It builds number sense and, crucially, lets a child judge whether an exact answer is reasonable: if 287 + 412 should be roughly 700, an answer of 1,099 is obviously wrong. The main techniques are rounding (adjust each number to the nearest 10, 100 or 1,000 and then compute), front-end estimation (use only the leading digits, 432 + 289 is about 400 + 200 = 600), and compatible numbers (nudge numbers to values that are easy to combine, 248 + 352 is about 250 + 350 = 600). The rounding rules are worth stating cleanly: to the nearest 10, look at the ones digit, 5 or more rounds up; to the nearest 100, look at the tens digit, 50 or more rounds up. Estimate 287 + 412 to the nearest hundred and you get 300 + 400 = 700, against an exact 699, close enough to confirm the real answer. The teacher's job is to weave estimation through everyday work, especially shopping and measurement contexts, so children reach for a ballpark figure before they trust an exact one. The symbol to know is the wavy equals, which reads as is approximately equal to.

✅ Solved examples

1. Estimate 287 + 412 by rounding to the nearest 100, then compare with the exact sum.
287 rounds to 300 and 412 rounds to 400, so the estimate is 300 + 400 = 700. The exact sum is 287 + 412 = 699, so the estimate of 700 is very close, confirming the answer is reasonable.
2. Estimate 876 - 249 by rounding to the nearest 10.
876 rounds to 880 and 249 rounds to 250, so the estimate is 880 - 250 = 630. The exact difference is 876 - 249 = 627, so 630 is a good estimate.
3. Ravi buys a book for Rs. 78 and a pen for Rs. 19. About how much does he spend?
Round to the nearest 10: 78 is about 80, 19 is about 20. The estimate is 80 + 20 = Rs. 100. The exact total is 78 + 19 = Rs. 97, so about Rs. 100 is a sound estimate for a quick shopping check.
4. Estimate 523 + 389. Is the true sum more than 900?
Round to the nearest 100: 500 + 400 = 900. The exact sum is 523 + 389 = 912, which is just above 900. So yes, the total is a little more than 900.

✏️ Practice — try these, take hints as needed

1. Estimate 612 + 295 by rounding to the nearest 100.
Round each number first.
612 rounds to 600, 295 rounds to 300.
About 900 (exact 907)
2. Round 749 to the nearest 100.
Look at the tens digit.
4 is less than 50.
700
3. Use front-end estimation for 432 + 289.
Use only the leading hundreds digits.
400 + 200.
About 600 (exact 721 with the rest; front-end leading estimate is 600)
4. Estimate 951 - 304 by rounding to the nearest 100, then give the exact answer.
951 to 1000, 304 to 300.
Then subtract the originals to check.
Estimate about 700; exact 647

📝 Topic test — 8 questions

Auto-graded with full solutions; saved to your dashboard. Use the calculator and formula sheet (top-right) any time.

Loading questions…
← Word Problems Take the chapter test →