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DECIMALS

COMMON MISTAKES

10/20/20091

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Fractions -Adding or Subtracting

How to Add or SubtractDecimals

Common Mistakes

To add or subtract decimal

numbers, you must align the

decimal point.

Not aligning the decimalpoint.

Incorrect

Correct

Note:

Add a 0 to align the decimal.

7.644.76

3.6

01.7

or3.6

01.7

++

13.3130.6

01.7

+

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Decimals - Multiplying

How to Multiply Decimals Common Mistakes

3

To multiply decimals,

multiply the numbers as

you would any pair of

numbers. Place the decimal point in

the final answeraccording

to the total number of

decimal places to the rightof the decimal point.

Not counting the correct

number of decimal places.

Counting from left to right.

Not placing a decimal at all. Incorrect: or

Correct: 4000

375

250

2.3

25.1

0.400

375

250

2.3

25.1

000.4

375

250

2.3

25.1

2 digits

1 digit

Total 3 digits

Count 3 digits from

right to left to place

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Decimals - Dividing

How to Divide Decimals Common Mistakes

4

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.

Not moving the decimal point thecorrect number of places or at all oneither the divisor or dividend or both.

Incorrect: Correct:

Note: The decimal was moved one place to

the right in both the divisor and dividend.

0

25

25

0

02

175

175

100

701.4

525.1175.2

2.5by117.525Divide

dividenddivisor

divisordividend

0

25

25

0

02

175

175

100

01.47

25.117525

2.5by117.525Divide

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Decimals Change Decimals to Fractions

How to Change Decimalsto Fractions Common Mistakes

5

To change a decimal to a fraction,

write the decimal without the

decimal point.

This number is the numerator of

the fraction.

The denominator will be 10, 100,

1000, etc. depending on the

number of decimal places of the

original decimal number.

One decimal place corresponds to

10, two decimal places

corresponds to 100, etc.

Setting the denominator by the

wrong number.

Incorrect:

Correct:

digit.oneis5

since10r todenominatoSetting:Mistake

10

50.05

fraction.ato0.05Change

places.decimaltwohasdecimal

originalsince100r todenominatosetMust

20

1

100

50.05

fraction.ato0.05Change

==

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