Mathematics
Grade 3
15 min
Estimate differences
Estimate differences
Tutorial Preview
1
Introduction & Learning Objectives
Learning Objectives
Round two-digit and three-digit numbers to the nearest ten.
Round three-digit numbers to the nearest hundred.
Use rounding to estimate the difference between two numbers.
Explain that an estimate is a 'close enough' answer, not an exact one.
Use estimation to check if a calculated answer is reasonable.
Solve word problems that ask for an estimated difference.
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In this lesson, you will learn a math superpower called 'estimation'. We will use rounding to find an answer that is close to the real one, which helps us do math in our heads much faster. This is a great way to check your work or get a quick idea of...
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Key Concepts & Vocabulary
TermDefinitionExample
EstimateAn estimate is a number that is close to the exact answer. It's a 'best guess' that you can find quickly.Instead of calculating 49 - 22 = 27, you can estimate that the answer is about 30.
DifferenceThe result of subtracting one number from another.In 10 - 4 = 6, the difference is 6.
RoundingChanging a number to a simpler 'friendly' number that is close to the original. We usually round to the nearest ten or hundred.The number 28 can be rounded to 30. The number 11 can be rounded to 10.
Nearest TenThe multiple of 10 (like 10, 20, 30...) that a number is closest to on a number line.The number 57 is closest to 60, so we round it to the nearest ten, which is 60.
Nearest HundredThe multiple of 100 (like 100, 200, 300...) that a number is c...
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Core Formulas
The Rounding Rule
Look at the digit to the right of the place you are rounding to. If the digit is 5, 6, 7, 8, or 9, you round UP. If the digit is 0, 1, 2, 3, or 4, you round DOWN.
Use this rule to decide whether to round a number up or down to the nearest ten or hundred. For example, to round 68 to the nearest ten, look at the 8. Since 8 is '5 or more', you round the 6 up to 7, making it 70.
The Estimation Process for Differences
Step 1: Round Number A \rightarrow Rounded A. Step 2: Round Number B \rightarrow Rounded B. Step 3: Rounded A - Rounded B = Estimated Difference.
This is the most important rule for estimating differences. Always round BOTH numbers FIRST, and only then do you subtract. Never subtract first.
4 more steps in this tutorial
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Challenging
The estimated difference between two numbers is about 60 when rounding to the nearest ten. Which of these could have been the original subtraction problem?
A.123 - 58
B.82 - 39
C.71 - 24
D.99 - 21
Challenging
A theater has 525 seats. 289 adults and 112 children are watching a movie. About how many seats are empty? (Estimate by rounding to the nearest hundred for all numbers).
A.200
B.100
C.124
D.0
Challenging
Leo tried to estimate 638 - 271 by rounding to the nearest ten. He got an answer of 470. What was his mistake?
A.He rounded 638 down to 630 instead of up to 640.
B.He rounded 271 up to 280 instead of down to 270.
C.He subtracted first and then rounded the answer.
D.He rounded both numbers to the nearest hundred instead of ten.
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