1. MSJC ~ San Jacinto Campus Math Center Workshop Series
DECIMAL NUMBERS
MSJC ~ San Jacinto Campus
Math Center Workshop Series
Janice Levasseur

2. MSJC ~ San Jacinto Campus Math Center Workshop Series
DECIMAL NUMBERS
MSJC ~ San Jacinto Campus
Math Center Workshop Series
Janice Levasseur

3. Introduction to Decimal Numbers
A number written in decimal notation has 3 parts:
Whole # part
The decimal point
Decimal part
The position of the digit in the decimal number determines the digit’s value.

4. Place Value Chart Whole number part Decimal part Decimal point . 103
102
101
100
10-1
10-2
10-3
10-4
10-5
tens
ones
tenths
thousands
hundreds
hundredths
thousandths
ten-thousandths
Hundred-thousandths
Whole number part
Decimal part
Decimal point

5. Writing a Decimal Number in Words
Write the whole number part
The decimal point is written “and”
Write the decimal part as if it were a whole number
Write the place value of the last non-zero digit
Ex: Write 6.32 in words
Six
and
thirty-two
hundredths

6. Ex: Write 0.276 in words Ex: Write 10.0304 in words Zero and
two hundred seventy-six
thousandths
Or two hundred seventy-six thousandths
Ex: Write in words
Ten
and
three hundred four
Ten-thousandths

7. Writing Decimal Numbers in Standard Form
Write the whole number part
Replace “and” with a decimal point
Write the decimal part so that the last non-zero digit is in the identified decimal place value
Note: if there is no “and”, then the number has no whole number part.

8. Ex: Write in standard form “seven hundred sixty-two thousandths”
Ex: Write in standard form “eight and three hundred four ten-thousandths” 8 .
Ex: Write in standard form “seven hundred sixty-two thousandths”
Note: no “and”  no whole part
0 .

9. Converting Decimal to Fractions
To convert a decimal number to a fraction, read the decimal number correctly. Simplify, if necessary.
Ex: Write 0.4 as a fraction
0.4 is read “four tenths” 
Ex: Write 0.05 as a fraction
0.05 is read “five hundredths” 

10. 0.007 is read “seven thousandths” 
Ex: Write as a fraction
0.007 is read “seven thousandths” 
Note: the number of decimal places is the same as the number of zeros in the power of ten denominator
Ex: Write 4.2 as a fractional number
Note: there’s a whole and decimal part 
Mixed number
4.2 is read “four and two tenths”  4

11. Your turn to try a few

12. Converting Fractions to Decimal Numbers (base 10 denominator)
When the fraction has a power of 10 in the denominator, we read the fraction correctly to write it as a decimal number
Ex: Write as a decimal number
The fraction is read “three tenths”
Note: no “and”  no whole part
0 . 3

13. . Ex: Write as a decimal number
The fraction is read “twenty-seven hundredths”
Note: no “and”  no whole part
0 .
Ex: Write as a decimal number
The mixed number is read “five and thirty-three thousandths” . 5

14. Converting fractions to decimals, take the numerator and divide by the denominator.
If the fraction is a mixed number, put the whole number before the decimal.
Rewrite as long division.

15. Ex: Write as a decimal number. 8 3 3 6 5
. 0
Place a bar over the part that repeats. 2
1 8
5/6
= 0.83 2
1 8 2
Is there an echo?
This will repeat  repeating decimal number

16. Ex: Convert to a decimal
Notice the mixed number – whole & fraction part 
The decimal number will have a whole & decimal part
The whole part is 2  2 . ________
Now convert the fraction 5/8 to determine the decimal part: . 6 2 5
2 5/8
= 2.625 8 5
. 0
4 8 2
1 6 4
4 0

17. Your turn to try a few

18. Rounding Decimal Numbers
Rounding decimal numbers is similar to rounding whole numbers:
Look at the digit to the right of the given place value to be rounded.
If the digit to the right is > 5, then add 1 to the digit in the given place value and zero out all the digits to the right (“hit”).
If the digit to the right is < 5, then keep the digit in the given place value and zero out all the digits to the right (“stay”).

19. Ex: Round 7.359 to the nearest tenths place
Identify the place to be rounded to:
Tenths
Look one place to the right. What number is there?
Compare the number to 5: 5 > 5  “hit” (add 1)
3 + 1 = 4 in the tenths place, zero out the rest
7.359 rounded to the nearest tenths place is
7.400 = 7.4

20. Ex: Round 22.68259 to the nearest hundredths place
Identify the place to be rounded to:
Hundredths
Look one place to the right. What number is there?
Compare the number to 5: 2 < 5  “stay” (keep)
Keep the 8 and zero out the rest
rounded to the nearest hundredths place is = 22.68

21. Ex: Round 1.639 to the nearest whole number
Identify the place to be rounded to:
ones
Look one place to the right. What number is there?
Compare the number to 5: 6 > 5  “hit” (add 1)
1 + 1 = 2 in the ones place, zero out the rest
1.639 rounded to the whole number is
2.000 = 2

22. Your turn to try a few

23. Decimal Addition & Subtraction
To add and subtract decimal numbers, use a vertical arrangement lining up the decimal points (which in turn lines up the place values.)
Ex: Add
put in 0 place holders
16.113
15.21 +
2.0036 3 3 . 3 2 6 6

24. put in the decimal point 16 . 0000 - 9.6413 put in 0 place holders
Ex: Subtract – 19.61 1 1 3 1
24.024
put in 0 place holders -
19.61 4 . 4 1 4
Ex: Subtract 16 – 1 9 9 9 5 1
put in the decimal point 16 .
0000 -
9.6413
put in 0 place holders 6 . 3 5 8 7

25. Your turn to try a few

26. Decimal Multiplication
Decimal numbers are multiplied as if they were whole numbers. The decimal point is placed in the product so that the number of decimal places in the product is equal to the sum of the decimal places in the factors.

27. Ex: Multiply 1.2 x 0.04
Think 12 x 4
 12 x 4 = 48
1.2 has 1 decimal place
0.04 has 2 decimal places
Therefore the product of 1.2 and 0.04 will have = 3 decimal places 48 .
 1.2 x 0.04 = 0.048

28. Ex: Multiply 3.1 x 1.45
Think 31 x 145
 31 x 145 =4495
3.1 has 1 decimal place
1.45 has 2 decimal places
Therefore the product of 3.1 and 1.45 will have = 3 decimal places .
 3.1 x 1.45 = 4.495

29. Multiply by Powers of 10 When multiplying by 10, 100, 1000, …
Move the decimal in the number to the right as many times as there are zeros.
2.345 times 10, move the decimal one place to the right, 23.45

30. Ex: Multiply x 10
Think x 10
 x 10 =
has 4 decimal place
10 has 0 decimal places
Therefore the product of and 10 will have = 4 decimal places
123450 .
 x 10 = =

31. Ex: Multiply x 100
Think x 100
 x 100 =
has 4 decimal place
100 has 0 decimal places
Therefore the product of and 100 will have = 4 decimal places .
 x 100 = =

32. Ex: Multiply x 1000
Think x 1000
 x 1000 =
has 4 decimal place
1000 has 0 decimal places
Therefore the product of and 1000 will have = 4 decimal places .
 x 1000 = =

33. So what have we seen?
x 10 =
1 zero  move decimal point 1 place to the right
x 100 =
2 zeros  move decimal point 2 places to the right
x 1000 =
3 zeros  move decimal point 3 places to the right
To multiply a decimal number by a power of 10, move the decimal point to the right the same number of places as there are zeros.

34. Ex: Multiply x 1000
How many zeros are there in 1000? 3
 Move the decimal point in to the right 3 times 34 . 31 .
 x 1000 = 34,310

35. Ex: Multiply 21 x 100
How many zeros are there in 100? 2
 Move the decimal point in to the right 2 times 21 . .
 21 x 100 = 2100

36. Your turn to try a few

37. Decimal Division
To divide decimal numbers, move the decimal point in the divisor to the right to make the divisor a whole number.
Move the decimal point in the dividend the same number of places to the right.
Place the decimal point in the quotient directly over the decimal point in the dividend.
Divide like with whole numbers.

38. Ex: Set up the division of 0.85 0.5. 5 . 8 5
Why does this work?
Multiplication Property of One, “Magic One”
Consider the fraction representation of the division:
Which is the equivalent division we get after moving the decimal point.

39. Ex: Divide 1 7 . . . 5 . 8 5 5 3 5
3 5

40. Ex: Set up the division . . 76 37 .
0 4 2 .

41. Ex: Divide 37.042 0.76, round to the nearest tenth.
When dividing decimals, we usually have to round the quotient to a specified place value.
Ex: Divide , round to the nearest tenth.
 the answer to the division (i.e. the rounded quotient) is 48.7 4 8 . 7 3 . 7 6 3 7 4 . 2
6 6 4
5 6 2
3 0

42. Divide by Powers of 10 When dividing by 10, 100, 1000, …
Move the decimal in the number to the left as many times as there are zeros.
76.89 divided 10, move the decimal one place to the left, 7.689

43. Ex: Divide 1 2 3 4 . 5 6 10 10 2 3 20 = 3 4 30 4 5 40 5 6 50 6

44. Ex: Divide 1 2 3 . 4 5 6
100
100 23 4 =
200 34 5
300 45 6
40 0
5 6
5 0 0
6 0

45. So what have we seen? =
1 zero  move decimal point 1 place to the left =
2 zeros  move decimal point 2 places to the left
To divide a decimal number by a power of 10, move the decimal point to the left the same number of places as there are zeros.