Counting in Groups, Rounding and Estimating

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What this lesson covers

  • Groups of 10 and 100 make big numbers quick to count.
  • Ten hundreds make one thousand.
  • To round to the nearest 100, look at the tens digit.
  • Rounding helps you estimate an answer before you work it out.
Key idea

Count big piles in groups

  • Counting 236 shells one at a time is slow, and it is easy to lose your place.
  • Put them in groups of 10. Then put ten groups of 10 together to make 100.
  • Count the hundreds, then the tens, then the ones.
GroupsWorth
2 hundreds200
3 tens30
6 ones6
Total236
Key idea

Try it: group into tens and hundreds

Worked example

Counting kiwifruit

  • A packhouse has 4 boxes of 100 kiwifruit, 7 bags of 10 and 5 loose ones.
  • 4 hundreds is 400. 7 tens is 70. Then 5 more.
  • 400 + 70 + 5 = 475 kiwifruit.
Key idea

Ten hundreds make a thousand

  • Count in hundreds: 100, 200, 300, 400, 500, 600, 700, 800, 900, 1,000.
  • That is ten hundreds. Ten hundreds is the same as one thousand.
Ten of theseMake one
onesten
tenshundred
hundredsthousand
Key idea

Rounding to the nearest 10

  • Rounding finds the nearest easy number.
  • To round to the nearest 10, look at the ones digit.
  • 0, 1, 2, 3 or 4: round down. 5, 6, 7, 8 or 9: round up.
NumberNearest 10
4340
4750
6570
238240
Key idea

Rounding to the nearest 100

  • To round to the nearest 100, look at the tens digit.
  • 0 to 4 in the tens: round down. 5 to 9 in the tens: round up.
  • On a number line, 362 sits between 300 and 400. It is closer to 400.
NumberTens digitNearest 100
3626400
4282400
7505800
6494600
Key idea

Try it: round a number

Worked example

Estimate before you work it out

  • Estimate 198 + 304.
  • 198 is about 200. 304 is about 300.
  • 200 + 300 = 500, so the answer is about 500.
  • The exact answer is 502. The estimate was close.

Common mistakes

  • 362 rounds to 300 because it starts with 3.
    Look at the tens digit. It is 6, so 362 rounds up to 400.
  • 45 rounds to 40.
    A 5 in the ones rounds up, so 45 rounds to 50.
  • 649 rounds to 700 because 9 is big.
    For the nearest 100, look at the tens digit. It is 4, so 649 rounds to 600.
  • An estimate is a wild guess.
    An estimate uses rounded numbers, so it lands close to the real answer.

Your turn

1

There are 3 boxes of 100 pens, 5 packs of 10 pens and 8 single pens. How many pens is that?

2

Round 76 to the nearest 10.

3

Round 451 to the nearest 100.

4

Round 349 to the nearest 100.

5

Estimate 402 + 297 by rounding each number to the nearest 100.

Show answer

1. 358 2. 80 3. 500 4. 300 5. About 700 (400 + 300). The exact answer is 699.

Summary

What to take away

  • Group in tens and hundreds to count big amounts.
  • Ten hundreds make a thousand.
  • Round to the nearest 100 by looking at the tens digit.
  • Round first to estimate, then check.
Report a mistake

More in Number

Numbers to 1,000: Hundreds, Tens and OnesNotesCounting in Groups, Rounding and EstimatingNotesSkip Counting, Odd and Even NumbersNotesAdding and Subtracting up to 1,000NotesMultiplying and Dividing with GroupsNotesUnit Fractions: One Equal PartNotesComparing Fractions and Finding Equal OnesNotesAdding Fractions and Finding Fractions of NumbersNotesDollars, Cents and Giving ChangeNotes

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