Transcript Compatible numbers

How do you
Estimate
Whole Numbers
Using Rounding
and Compatible
Numbers?
Sometimes in math you do not need an
exact answer. Instead, you can use an
estimate. Estimates are close to the exact
answer but are usually easier and faster to
find.
When estimating, you can round the
numbers in the problem to compatible
numbers. Compatible numbers are close
to the numbers in the problem, and they
can help you do math mentally.
Remember!
When rounding, look at the digit to the
right of the place to which you are
rounding.
• If that digit is 5 or greater, round up.
• If that digit is less than 5, round down.
Estimate the sum by rounding to the place
value indicated.
12,345 + 62,167; ten thousands
10,000
+__________
60,000
70,000
Round 12,345 down.
Round 62,167 down.
The sum is about 70,000.
Estimate the difference by rounding to the
place value indicated.
4,983 – 2,447; thousands
5,000
–
2,000
__________
3,000
Round 4,983 up.
Round 2,447 down.
The difference is about 3,000.
Estimate the sum by rounding to the place
value indicated.
13,235 + 41,139; ten thousands
10,000
+__________
40,000
50,000
Round 13,235 down.
Round 41,139 down.
The sum is about 50,000.
Estimate the difference by rounding to the
place value indicated.
5,723 – 1,393; thousands
6,000
–
1,000
__________
5,000
Round 5,723 up.
Round 1,393 down.
The difference is about 5,000.
An estimate that is less than the exact
answer is an underestimate.
An estimate that is greater than the
exact answer is an overestimate.
Chelsea is planning the annual softball
banquet for the 8 teams in the region. Each
team has 18 members. Estimate how many
plates she will need to buy if all the members
attend.
Find the number of softball members.
8  18
8  20
8  20 = 160
Overestimate the number of
softball members.
The actual number of softball
members is less than 160.
Chelsea should buy about 160 plates.
Another method
Find the number of softball members.
10  18
10  18 Overestimate the number of
teams.
10  18 = 180
The actual number of softball
members is less than 180.
Chelsea should buy about 180 plates.
Ms. Oliver wants to buy the entire seventhgrade new pencils. There are 5 seventh-grade
homeroom classes of 28 students. Estimate
how many pencils Ms. Oliver needs to buy for
all of the students.
Find the number of students in the seventh grade.
5  28
5  30
5  30 = 150
Overestimate the number of
students.
The actual number of students is
less than 150.
Ms. Oliver should buy about 150 pencils.
Estimating a Quotient Using Compatible
Numbers
Mr. Douglas will drive 243 miles to the fair at
65 mi/h. About how long will his trip take?
243 ÷ 65
240 ÷ 60
240 ÷ 60 = 4
240 and 60 are compatible
numbers. Underestimate
the speed.
Because he underestimated
the speed, the actual time
will be less than 4 hours.
The trip will take about 4 hours.
Mrs. Blair will drive 103 miles to the airport at
55 mi/h. About how long will her trip take?
103 ÷ 55
100 ÷ 50
100 and 50 are compatible
numbers. Underestimate
the speed.
Because she underestimated
100 ÷ 50 = 2 the speed, the actual time will
be less than 2 hours.
The trip will take about 2 hours.
Lesson Quiz
Estimate each sum or difference by rounding
to the place value indicated.
1. 7,420 + 3,527; thousands
2. 47,821 + 19,925; ten thousands
3. 8,254 – 5,703; thousands
4. 66,845 – 24,782; ten thousands
5. One quart of paint covers an area of 100 square
feet. How many quarts are needed to paint a
wall 8 feet tall and 19 feet wide?
Lesson Quiz Answers
Estimate each sum or difference by rounding
to the place value indicated.
1. 7,420 + 3,527; thousands
11,000
2. 47,821 + 19,925; ten thousands 70,000
3. 8,254 – 5,703; thousands
2,000
4. 66,845 – 24,782; ten thousands 50,000
5. One quart of paint covers an area of 100 square
feet. How many quarts are needed to paint a
wall 8 feet tall and 19 feet wide? 2