rounding and estimating -...
TRANSCRIPT
Rounding and Estimating
Learning Objective:To be able to round numbers to the nearest 10 or 100.
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Sometimes in maths we don’t need the exact answer. We can ROUND numbers then add them together to give us a close
answer. We can round numbers up or down.
Do you know the rounding rule?
Think, pair, share your ideas.
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Both of these doors lead to the same room. If you were the robot, which door would you go through?
I would go through the door on the left because it’s the door
closest to me.
These numbers would all round down to 20 because they are closer to 20 than 30.
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The same rule applies to rounding. If you are rounding to the nearest ten, you go to the ten the number is closest to.
20 30
24232221
These numbers would all round up to 30 because they are closer to 30 than 20.
20 30
29282726
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20 30
25
The number 25 is in the middle of 20 and 30. Should we round up
or round down?
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These numbers would all
round down. Can you explain why?
These numbers would all round up. Can you
explain why?
Rounding Rules:
If the number ends in 1, 2, 3 or 4 you round down.
If the number ends in 5, 6, 7, 8 or 9 you round up.
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20 30
25
If a number has a ‘5’ in the ones column, you ALWAYS round UP instead of down. 25 would round up to 30.
13
64142
491821873 27
15255
328992546
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What is this number rounded to the nearest
ten?
18
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Well done! You would round 18 UP to 20.
10 20
18
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What is this number rounded to the nearest
ten?
61
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Well done! You would round 61 DOWN to 60.
60 70
61
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What is this number rounded to the nearest
ten?
45
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Well done! You would round 45 UP to 50.
40 50
45
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What is this number rounded to the nearest
ten?
36
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Well done! You would round 36 UP to 40.
30 40
36
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What is this number rounded to the nearest
ten?
94
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Well done! You would round 94 DOWN to 90.
90 100
94
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Good job! You have found out how to round numbers to the nearest 10.
Did you know you can also round numbers to the nearest 100? How do
you think you can do this?
These numbers would all round up. Can you
explain why?
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When you are rounding a number to the nearest hundred, the important number to look at is the number in the tens column, NOT the number in the ones column. The same rounding rule
applies: if the digit in the tens column is 0, 1, 2, 3 or 4 you round down; if the digit in the tens column is 5, 6, 7 or 8 you round up.
These numbers would all
round down. Can you explain why?
138
249208
3166361231 276
152391
4827647192
300 400
326
300 400
351
300 400
382
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What is this number rounded to the nearest
hundred?
172
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Well done! You would round 172 UP to 200.
100 200
172
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What is this number rounded to the nearest
hundred?
836
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Well done! You would round 836 DOWN to
800.
800 900
836
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What is this number rounded to the nearest
hundred?
719
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Well done! You would round 719 DOWN to 700.
700 800
719
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What is this number rounded to the nearest
hundred?
253
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Well done! You would round 253 UP to 300.
200 300
253
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What is this number rounded to the nearest
hundred?
392
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Well done! You would round 392 UP to 400.
300 400
392
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Great work everyone! Are you
ready to try rounding on your own?