DECIMALS
COMMON MISTAKES
1
10/20/2009
Fractions -Adding or Subtracting
How to Add or Subtract
Decimals

To add or subtract decimal
numbers, you must align the
decimal point.
Common Mistakes

Not aligning the decimal
point.

Incorrect
7.01
+ 6.3
or
76.4

Correct
7.01
+ 6.3
7.64
7.01
+ 6.30
13.31
Note:
Add a 0 to align the decimal.
2
Complete Manual: ..\Decimal Number Review.docx
To view; right click and open the hyperlink
10/20/2009
Decimals - Multiplying
How to Multiply Decimals
To multiply decimals,
multiply the numbers as
you would any pair of
numbers.
Place the decimal point in
the final answer according
to the total number of
decimal places to the right
of the decimal point.


Common Mistakes
 Not counting the correct
number of decimal places.
 Counting from left to right.
 Not placing a decimal at all.
 Incorrect: 1.25 or 1.25
× 3.2
× 3.2
375
250
375
4000
400.0
250

Correct:
1.25
× 3.2
250
375
4.000
3
Complete Manual: ..\Decimal Number Review.docx
To view; right click and open the hyperlink
←2 digits
←1 digit
←Total 3 digits
←Count 3 digits from
right to left to place
decimal point.
Decimals - Dividing
How to Divide Decimals



To divide decimals, the divisor
must contain no decimals.
Move the decimal point to the
right until divisor is a whole
number.
Move the decimal point on the
dividend by the same number of
places as the divisor.
dividend ÷ divisor
divisor dividend
Common Mistakes


Not moving the decimal point the
correct number of places or at all on
either the divisor or dividend or both.
Incorrect:
Correct:
Divide 117.525 by 2.5
4.701
2.5 117.525
100
175
175
02
0
25
25
0
Divide 117.525 by 2.5
47.01
25 1175.25
100
175
175
02
0
25
25
0
Note: The decimal was moved one place to
the right in both the divisor and dividend.
4
Complete Manual: ..\Decimal Number Review.docx
To view; right click and open the hyperlink
Decimals – Change Decimals to Fractions
How to Change Decimals
to Fractions

Common Mistakes
Setting the denominator by the
wrong number.
Incorrect:
To change a decimal to a fraction,
write the decimal without the

decimal point.
Change 0.05 to a fraction.
5
This number is the numerator of
0.05 ≠
10
the fraction.
Mistake : Setting denominator to 10 since
The denominator will be 10, 100,
5 is one digit.
1000, etc. depending on the
number of decimal places of the
 Correct:
original decimal number.
Change 0.05 to a fraction.
One decimal place corresponds to
5
1
0.05 =
=
100 20
10, two decimal places
Must set denominator to 100 since original
corresponds to 100, etc.




decimal has two decimal places.
5
Complete Manual: ..\Decimal Number Review.docx
To view; right click and open the hyperlink