Before you work out an exact answer, you can get a quick about right one by rounding each number first. Round numbers are easy to add or multiply in your head, so the estimate takes a moment — and it tells you roughly what the real answer should be.

Take 38 + 43. Round each to the nearest ten: 38 \to 40 and 43 \to 40. Now the sum is easy — 40 + 40 = 80 — so:

The wavy \approx sign means approximately equal to: not exactly, but close. The true total is 81, and our estimate of 80 sits right next to it.

Press play, then replay it: a sum like 38 + 43 appears, each number snaps to its nearest ten on the number line, and the easy estimate lands beside the true total so you can see how close they are.

The same trick works for multiplication. To estimate 19 \times 21, round to 20 \times 20 = 400:

The exact answer is 399 — so the estimate is a great match, and far easier to do in your head.

An estimate is also a friendly check on an exact answer. If you add 38 + 43 and get 811, your estimate of 80 shouts that something is wrong — the real answer should be about 80, not ten times bigger. A quick estimate catches silly slips before they cause trouble.

Khan Academy walks through using rounding to estimate sums here: