Estimating/
Rounding
Standard: 3.A.1.1
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Concept Skill:
Estimating/Rounding
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To estimate means to find an answer that is close to an
exact amount.
To round means to find about how many or how much by
expressing a number to the nearest ten, hundred, thousand,
and so on.
On way of estimating is by looking at a picture without
counting each object. You can estimate for math problems
by comparing one diagram to another.
When rounding, find the place value you are rounding to. If
you are rounding to the nearest ten, find the tens place. If
you are rounding to the nearest hundred, find the hundreds
place.
Look at the place value to the right. If you are rounding to
the nearest ten, look at the ones place. If you are rounding
to the nearest hundred, look at the tens place.
If that digit is 0, 1, 2, 3, or 4, round down. If that digit
is 5, 6, 7, 8, or 9, round up.
For example, to round 73 to the nearest ten:
Find the place value you are rounding to. You are rounding
to the nearest ten, so find the tens place: 73.
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Look at the place value to the right. You are rounding to the
nearest ten, so look at the ones place: 73.
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That digit is a 3. That means that you should round down to
the nearest ten. The nearest ten is 70, so round 73 down to
70.
You can round numbers to help you solve problems. For
example, adding 49 and 22 is difficult to do in your head. If
you round the numbers first, you can find an estimated
answer that is close to the exact answer.
“Teacher Sample Questions”
Example: 1
Melanie and Desiree both have containers with
pencils.
Melanie’s
Desiree’s
Does Melanie have less than, more than, or the same
number of pencils as Desiree?
Step 1: Compare the two containers/diagrams.
(The first looks like it is completely full. The second diagram
looks almost empty.
Step 2: Decide how the pencils compare.
(Melanie’s container is fuller than Desiree’s. Melanie has
more pencils than Desiree).
Example: 2
Estimate 138 + 73
Step 1: Round the first number to the nearest ten.
(Find the tens place: 138).
Look at the place value to the right, the ones place: 128.
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Step 2: Decide whether to round up or down.
(The digit in the ones place is 8, so I have to round up).
138 rounds up to 140
Step 3: Round the second number to the nearest ten.
(Find the tens place: 73).
Look at the place value to the right, the ones place: 73.
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Step 4: Decide whether to round up or down.
(The digit in the ones place is 3, so I have to round down.
73 rounds down to 70
Step 5: What is your estimated answer?
(A good estimate for 138+73 is 140+70=210
“STUDENT PRACTICE”
If the length of the car shown above is about nine
feet, the length of the van is about
A.
B.
C.
D.
eight feet
twelve feet
eighteen feet
twenty feet
Felix wants to add 78 and 37. Which of the following
would be best for Felix to use to estimate the sum?
A.
B.
C.
D.
80 + 30
90 + 20
80 + 40
90 + 40
A ticket to the dolphin show costs $18 for 1 adult.
Which is the closest to the total cost of tickets for 4
adults?
A.
B.
C.
D.
$20
$40
$80
$120
“KEY”
If the length of the car shown above is about nine
feet, the length of the van is about
A.
*B.
C.
D.
eight feet
twelve feet
eighteen feet
twenty feet
Felix wants to add 78 and 37. Which of the following
would be best for Felix to use to estimate the sum?
A.
B.
*C.
D.
80 + 30
90 + 20
80 + 40
90 + 40
A ticket to the dolphin show cost $18 for 1 adult.
Which is the closest to the total cost of tickets for 4
adults?
A.
B.
*C.
D.
$20
$40
$80
$120
“STUDENT PRACTICE”
If the stack of books shown above is 2 feet high, then
about how tall is the light pole?
A.
B.
C.
D.
3 feet
6 feet
12 feet
20 feet
The school auditorium has 99 seats. People are
sitting in 68 of the seats. Which is the best estimate
of the number of seats that do not have people sitting
in them?
Use words, numbers, or pictures to show your answer.
“KEY”
If the stack of books shown above is 2 feet high, then
about how tall is the light pole?
A.
B.
*C.
D.
3 feet
6 feet
12 feet
20 feet
The school auditorium has 99 seats. People are
sitting in 68 of the seats. Which is the best estimate
of the number of seats that do not have people sitting
in them?
Use words, numbers, or pictures to show your answer.
99 rounds to 100
68 rounds to 70
100 – 70 = 30