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← Level 2 – Times-table kid

17 / 30 ⏱ 14 minutes

Rounding

How to round to tens and hundreds and what it is good for.

Round: roughly, but smart

How many people were at the football? 18 347. Nobody will remember that. But “about 18 thousand” everyone will remember.

Rounding is swapping an exact number for a nearby round one. A round number ends in a zero (or more zeros) and the head works with it much better.

You round to tens (47 → 50), to hundreds (368 → 400), later to thousands too. You always look for the nearest round number: 47 sits between 40 and 50 and it is closer to fifty — three steps instead of seven.

And what about a number exactly in the middle, say 45? A worldwide agreement: a five rounds up. 45 → 50.

You will need rounding right in the next lesson for estimates — and in life all the time: how much shopping costs, how many people will come, how much time is left.

Sofie reads the news with dad: “The concert was visited by 9 823 people.” Dad asks: “About how many is that?” Sofie thinks: 9 823 is between 9 000 and 10 000… and closer to ten thousand. “About ten thousand.” The next day at school she talks about a concert for ten thousand people and nobody asks for the exact number. In the evening she notices a price tag 397 Kč and says by herself: “That is almost four hundred.” Dad smiles — Sofie started rounding without being asked.

💡

The digit one floor down decides, from the one you round to. To tens? Look at the ones. To hundreds? Look at the tens. There is 0–4 → down, 5–9 → up.

Method: find the neighbours, measure the distance

Rounding to tens, say 47:

  1. Find the round neighbours: 47 sits between 40 and 50.
  2. Look at the ones: 7.
  3. Decide: 7 is in the 5–9 gang → up. 47 → 50.

Write-up with an arrow or a dot: 47 ≐ 50 (the sign ≐ means “approximately equal”).

Rounding to hundreds, say 368:

  1. Neighbours: 300 and 400.
  2. The tens digit decides: 6.
  3. 6 → up: 368 ≐ 400.

Watch: the ones (that eight) do not interest you at all when rounding to hundreds. You look only one floor down.

In the middle: 45 to tens → the one is 5 → up → 50. 250 to hundreds → the ten is 5 → up → 300.

A round number does not change: 80 to tens stays 80. It is already round.

One important thing: a rounded number is a stand-in, not the truth. It is good for estimates and talking. At the till, in a recipe and in a test the exact number belongs — unless the question says “round”.

To tens

Round to tens: 63, 78, 45.

63: neighbours 60 and 70, one 3 → down → 60.

78: neighbours 70 and 80, one 8 → up → 80.

45: neighbours 40 and 50, one 5 → by agreement up → 50.

A quick sense check: 63 really is closer to sixty (3 steps) than to seventy (7 steps). The 0–4/5–9 rule only shortens this measuring.

To hundreds

Round to hundreds: 368, 349, 750.

368: the ten 6 decides → up → 400.

349: the ten 4 decides → down → 300. (Even though the one is 9! That one does not speak here.)

750: the ten 5 decides → up → 800.

Notice 349: it is only one less than 350, and yet it falls down to 300. The border splits exactly in the middle — 350 and up goes up.

Price tags in a shop

Mum sent you shopping: milk 23 Kč, bread 38 Kč, cheese 47 Kč. Is a hundred enough?

I round to tens: 23 ≐ 20, 38 ≐ 40, 47 ≐ 50.

I add the round ones: 20 + 40 + 50 = 110.

Hm, 110 > 100 — a hundred is on the edge. And watch, I rounded the milk down, so the real total can be even more than the estimate. Exactly: 23 + 38 + 47 = 108 Kč. A hundred is not enough. The estimate warned me correctly.

⚠️

Look only one floor down. When rounding 349 to hundreds, the four of the tens decides (down to 300) — the nine of the ones does not speak. Whoever rounds twice (349 → 350 → 400) gets the wrong result.

Practice: now you do it 🎲

Now try it for real. You will see all the questions at once, like on paper. 开始 with ten. When it goes well, add more.

Round to tens. The ones decide: 0–4 down, 5–9 up.

Practise the questions

On paper

Draw number lines — on them rounding is visible.

  1. Draw a line from 40 to 50 with marks of one. Mark 47 and with an arrow show which end it is closer to.

  2. Round to tens: 34, 85, 91, 55, 60.

  3. Round to hundreds: 234, 468, 650, 505, 96.

  4. Find all the numbers that round to 70 to tens. (Hint: there are ten of them, from 65 to 74.)

Now you 💪

  1. You know the rule: 0–4 down, 5–9 up — and you know that the digit one floor down decides.
  2. You round 349 to hundreds correctly to 300, not to 400.
  3. You can say what rounding is for and when you want the exact number instead.
✅

Done when: You round every number to tens and hundreds and you know which digit decides.

What to take from this lesson

© 2026 Ing. Martin Polak / AlgoRhino · 内容使用条款