Adding and Subtracting Decimals

							Adding and Subtracting
Decimals
            NS11



                         1
 Recall that when we add or subtract
  money, we write the numbers so that the
  decimal points are vertically aligned.
 This way we are sure to add digits with the
  same place value.
 We insert the decimal point in the answer
  below the other decimal points, as shown
  above.

                                            2
 We use the same procedure to add or
  subtract any decimal numbers.
 We keep the decimal points in line.
 This way, we add or subtract digits with
  the same place value.
 The decimal points stay in a straight line,
  as shown above.

                                                3
Let’s practice!
   Find the perimeter of the
    triangle. Units are in
    centimeters.
   How do we find perimeter?
   We keep the decimal points
    aligned in the problem so we
    can add digits with the same
    place value.
   We add the digits column by
    column, just as we would add
    whole numbers or money.

                                   4
Another Example:
   The roof was 6.37 meters above
    the ground. The ladder could
    reach only 4.2 meters. The roof
    was how much higher than the
    ladder could reach?
   We line up the decimal points
    vertically so that we subtract
    digits with the same place value.
   We attach a zero to 4.2 so that
    there are no empty places in the
    problem. Then we subtract.
                                        5
Add: 6.2+3+ 4.25
   To add digits with the same place
    value, we align decimal points.
   In this problem the whole number 3
    has the same place value as the 6
    and the 4.
   We place a decimal point to the
    right of the 3 and align decimal
    points.
   We may fill empty decimal places
    with zeros if we wish.


                                         6
   For some subtraction
    problems we need to
    add decimal places to
    perform the
    subtraction.
   If we subtract 0.23
    from 0.4, we find
    there is an “empty”
    place in the problem.
   We fill the empty
    place with a zero.
    Then we subtract.

                            7
Subtract: 0.4 – 0.231
   We set up the
    problem by lining up
    the decimal points,
    remembering to write
    the first number on
    top.
   We fill empty places
    with zeros.
   Then we subtract.

                           8
Subtract: 3 – 1.23
   This problem is
    similar to subtracting
    $1.23 from $3.
   We place the decimal
    point to the right of
    the 3, fill the decimal
    places with zeros,
    and subtract.


                              9
4.3 + 2
a.   4.5
b.   6.3
c.   4.1
d.   2.3

           10
4 + 1.3 + 0.6
a. 2.3
b. 5.3
c. 5.9
                11
4.75 – 2
a. 2.75
b. 4.73
c. 6.75
d. 4.77
           12
0.3 – 0.15
a. 0.25
b. 0.15
c. 0.12
d. 0.18
             13
1 – 0.1
a. 0.9
b. 1.1
c. 0

          14
2.5 – 1
a. 2.4
b. 2.6
c. 1.6
d. 1.4
          15
1 – 0.25
a. 0.24
b. 0.75
c. 1.25
d. 0.15
           16
The end!
      17