Solving Decimal Problems using Estimation
Study Sheet
Solve the following questions by estimation using decimal rounding. Show full working.
Next to your working explain what you did and your thinking.
1 Mr Lewis bought 15.8 pounds of
pork chops on sale for a total of
$35.49. Which of these is the
best estimate of the price per
pound
A.
B.
C.
D.
$2.00
$3.00
$4.00
$5.00
2 Which of these is the
estimate of 0.47 x 126.4?
A.
B.
C.
D.
best
30
40
50
60
3 A writer kept track of the time he
spent on a project. His notes are
show in the table.
Day
Mon.
Tues.
Wed.
Thurs.
Fri.
Time
(in hours)
2.9
4.25
5.75
7.3
4.9
The writer spent 9% of his time on
the phone. Which of these is the
best estimate of the amount of
time he spent on the phone?
A.
B.
C.
D.
less than 2 hours
between 2 and 2.5 hours
between 2.5 and 3 hours
more than 3 hours
Created by David Ritchie, A.U.S.S.I.E. Mathematics Consultant
Solving Decimal Problems using Estimation
Study Help
Reading the question
Underline or circle the key (important) words. If you wish cross out the unnecessary words
Look for the words related to accuracy: nearest unit, nearest tenth.
Be careful of the words “best estimate”. This normally means there is one answer that is
better than the rest.
Basic Facts -Remember common percentage, fraction and decimal equivalents.
Percentage
Fraction
out of 100 and reduced
Decimal
25%
50%
75%
10%
20%
5%
1%
Dividing by a number and Multiplying by a Fraction
Multiplying by ½
Finding ½ of an amount
is the same as
is the same as
is the same as
is the same as
is the same as
Finding 1/10 of a number
Dividing by ⅓
Dividing by 0.5
Dividing by 2
Dividing by 2
Dividing by 10
Multiplying by 3
Multiplying by 2
Rounding – be careful of the words “best estimate”
Be careful. Don’t just think you can round up or down to the nearest unit, or 10.
For example: 36.8 ÷ 0.61, could have different answer depending on the rounding
technique used.
Consider, which is the best estimate
Possible thinking


A.
B.
37
36
37 ÷ 1
36 ÷ 1

C.
74
37 ÷ 0.5 = 74

D.
60
36 ÷ 0.6 = 60
= 37
= 36
Error analysis
(under-estimate as you have rounded 0.61 up to 1)
(under-estimate as you have rounded 0.61 up to 1 and rounded
36.8 down to 36)
(over-estimate as you have rounded 0.61 down to 0.5, and say how
many ½ in 37)
(this is the best estimate, how many 0.6s in 36, is 10x how many
6s in 36.)
Created by David Ritchie, A.U.S.S.I.E. Mathematics Consultant
Solving Decimal Problems using Estimation
Teacher Notes
It is important to get student to round accurately and use estimation in order to reduce the
time spent doing these type of questions. Less time spent on estimation questions will
allow more time for complicated questions.
1




2




3
Mr Lewis bought 15.8 pounds of pork
chops on sale for a total of $35.49.
Which of these is the best estimate
of the price per pound
A.
B.
C.
D.
$2.00
$2.50
$3.00
$3.50
30
40
50
60
A writer kept track of the time he
spent on a project. His notes are
show in the table.
Day
Mon.
Tues.
Wed.
Thurs.
Fri.
Time
(in hours)
2.9
4.25
5.75
7.3
4.9
The writer spent 9% of his time on
the phone. Which of these is the
best estimate of the amount of time
he spent on the phone?




A.
B.
C.
D.
36 ÷16 = 2 and 6/16
6/16 is just less than 8/16 which is ½
so $2.50
Which of these is the best estimate
of 0.47 x 126.4?
A.
B.
C.
D.
If you use rounding to nearest unit,
less than 2 hours
between 2 and 2.5 hours
between 2.5 and 3 hours
more than 3 hours
Use rounding principle
Recognise 0.47 ≈ 0.5
That 0.5 = ½
126.4 can be rounded down to 120, so ½ of
120 is 60.
If you increased 126.4 up to 130 and used
0.5, you would get 65 which would be an
overestimate.
Total the hours
Day
Mon.
Tues.
Wed.
Thurs.
Fri.
TOTAL
Time
(in hours)
2.9
4.25
5.75
7.3
4.9
25.10
Use 10% instead of 9%.
(so your answer will be an overestimate)
10% = 10 = 1
100 10
To find 1 divided by 10.
10
so 1 of 25 is 2.5 (so 2.5 hours).
10
As this is an overestimate the answer is
between 2 and 2.5 hours.
Created by David Ritchie, A.U.S.S.I.E. Mathematics Consultant