Estimation checks whether answers are reasonable. Round each number to the nearest ten and perform the operation mentally. Example: 24 + 18 ≈ 20 + 20 = 40, so an exact answer near 42 is reasonable.
You already know how to round numbers to the nearest ten — a number like 24 rounds to 20 because it's closer to 20 than to 30. Estimation puts that rounding skill to a new purpose: instead of finding an exact answer, you find a quick, close-enough answer that tells you whether your exact calculation makes sense.
Here's why estimation matters. Imagine you add 47 + 38 and get 125. Does that seem right? If you round first — 50 + 40 = 90 — you immediately know 125 is way off. You caught the error without checking every step. Estimation is a self-checking tool that makes you a more confident mathematician.
The process has two steps: round, then operate. For 24 + 18, round each number to the nearest ten to get 20 + 20 = 40. For 63 − 29, round to get 60 − 30 = 30. You don't need pencil and paper for these — the rounded numbers are easy enough to compute in your head. This is the same mental math you practiced with tens and hundreds.
An important word in estimation is reasonable. When you get an exact answer, ask: is this close to my estimate? If your estimate was 40 and your exact answer is 42, that's reasonable — they're close. If your exact answer is 82, that's not reasonable — something went wrong. Estimation doesn't replace exact computation, but it gives you a target range so you know when an answer is worth trusting.