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ELI5 Yahaaa

Close enough on purpose

What Is Estimating?

An estimate is a deliberately rough answer that is fast and good enough.

Exact Slow, precise,sometimes unnecessaryEstimate Fast, roughly right,usually enough
Choosing which one you need is a skill.

  1. Step 1 of 5

    Sometimes exact is a waste of effort.

    The exact total?£17.43 — takes realworkThe estimate About £17 — takesseconds

    Deciding if your shopping fits a £20 note does not need the exact total. You need to know whether it is over or under.

  2. Step 2 of 5

    Round the numbers to friendly ones.

    1 £3.95 + £2.10 +£5.802 Round: 4 + 2 +63=About £12

    £3.95 is about £4. £2.10 is about £2. Rounding to whole pounds turns a hard sum into one you can do while walking.

  3. Step 3 of 5

    Know which way you rounded.

    Rounded up Estimate is above thetrue valueRounded down Estimate is below it

    If you rounded everything up, your estimate is too high and the real answer is lower. That tells you how much to trust it.

  4. Step 4 of 5

    Estimating catches mistakes.

    1?Calculator says3,9902 But 20 × 20 =4003−Something iswrong

    If you calculate 19 × 21 and get 3,990, an estimate of 20 × 20 = 400 tells you instantly that something went wrong.

  5. Step 5 of 5

    Some questions only ever get estimates.

    Hairs on a headabout 100,000Leaves on an oakabout 200,000Blades of grass on a lawnmillions

    How many hairs on your head? How many leaves on a tree? Nobody counts these — a good estimate is the only sensible answer.

    Physicists call these Fermi problems, after a scientist who estimated a nuclear blast's power by dropping paper scraps.

The short version

An estimate trades precision for speed — and it is often the better answer, plus a check on the exact one.

Try it yourself

Guess how many words are on a page, then count one line and multiply. Your quick estimate is usually very close.

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