Mathematics
Grade 3
15 min
Estimate sums
Estimate sums
Tutorial Preview
1
Introduction & Learning Objectives
Learning Objectives
Define estimation and explain why it is useful.
Round two-digit numbers to the nearest ten.
Round three-digit numbers to the nearest ten and nearest hundred.
Use rounding to estimate the sum of two- and three-digit numbers.
Use estimation to check if an exact answer is reasonable.
Use estimation vocabulary like 'about', 'close to', and 'approximately'.
You have 58 stickers and your friend gives you 31 more. About how many stickers do you have in total? 🤔 Let's find a super-fast way to know!
In this lesson, we will learn how to estimate sums. Estimating is like making a smart guess to find an answer that is 'close enough' to the real one. It's a math superpower that helps you do math quickly in your head!...
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Key Concepts & Vocabulary
TermDefinitionExample
EstimateTo find an answer that is close to the exact answer. It's not perfect, but it's nearby and easier to find.The exact sum of 21 + 29 is 50. An estimate could be 20 + 30 = 50.
SumThe answer you get when you add two or more numbers together.In 7 + 5 = 12, the number 12 is the sum.
RoundingChanging a number to a simpler, 'friendly' number that ends in zero. This makes it easier to do math in your head.The number 28 is close to 30, so we can round 28 to 30.
AboutA word we use to show that our answer is an estimate, not an exact amount.22 + 49 is about 70.
Place ValueThe value of a digit based on its position in a number, like the ones, tens, or hundreds place.In the number 472, the 4 is in the hundreds place, the 7 is in the tens place, and the...
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Core Formulas
The Rounding Rule
1. Find the place. 2. Look next door. 3. Five or more, raise the score. 4. Four or less, let it rest.
Use this rhyme to remember how to round. 'Raise the score' means add one to the digit in the place you are rounding to. 'Let it rest' means keep that digit the same. All digits to the right become zeros.
The Estimation Formula
a + b \approx \text{round}(a) + \text{round}(b)
This formula shows that to estimate a sum, you first round the numbers you are adding (a and b), and then you add the rounded numbers together. The 'wavy' equals sign (≈) means 'approximately equal to'.
4 more steps in this tutorial
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Challenging
The estimated sum of two numbers, when rounded to the nearest ten, is 90. Which of these could be the original numbers?
A.55 + 45
B.52 + 39
C.41 + 42
D.59 + 38
Challenging
A farmer picked 121 apples, 288 peaches, and 309 pears. About how many pieces of fruit did he pick in all? (Estimate by rounding to the nearest hundred).
A.700
B.600
C.720
D.800
Challenging
To estimate 152 + 239, Sam rounds to the nearest hundred (200 + 200 = 400). Maria rounds to the nearest ten (150 + 240 = 390). The exact answer is 391. Who is correct?
A.Only Sam is correct.
B.Only Maria is correct.
C.Neither is correct because they didn't find the exact answer.
D.Both are correct because they both used a valid estimation strategy.
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