Download multiplying decimals

Multiplication: numbers with decimals:
Big Idea: Again, we often multiply decimals in our daily lives. John earns $8.50 per hour. How
much will he earn after 4 hours of work?
$8.50
X 4
$34.00
Note: we did not line the decimal up. We don’t when we multiply or divide decimals. In stead, we
solve the problem as if it were a whole-number problem. We then count the total number of digits
to the right of the decimal point in both factors (numbers we are multiplying). We then place the
decimal point in the answer so that there is the same number of digits to the right of the decimal
point.
Model:
1
x
2 2
1 1
1 1
1 5 3
1 4 2
$12.45 2 digits to the right
12 of the decimal
249 0
1245 0
149.40
21 475
345.7
x.42 2
691 4
6914 0
257 700
138280 0
279.175
145.8854
X
$42.9 5
6.5
1
Lesson:
$15.25
x
.3
0.2 X 3 =
129.5
X
.65
0.5 X 0.5 =
3.457
x .32
.2 X .5 =
.4 X .4 =
Practice: In pairs, solve the following questions and compare answers.
1)
1.1
x13
2)
.1 4
x 1.5
3)
.15
X 15
Lock Down What You Know: solve the following multiplication questions with decimals.
Remember to ignore the decimal until you have completed solving the question and then count
the number of digits in the question that are on the right-hand side of the decimal.
Just two questions across your page please.
Do a – e without rewriting vertically. Solve and then place the decimal.
a) .5 X .4 =
b) .2 X . 9 =
f)
g)
1.2
x .1 2
k) 9.3
x 6.3
l)
.11
x .11
.68
x 42
c) .8 X .3 =
h) 3.6
X36
m) 2.26
x 1.2
d) .9 X .3 =
i)
.76
X 22
n) .24
x .38
e) .1 X .234 =
j)
92
x 4.5
o)
3.6
X 1.6
For extra help, go to the following websites
www.learnalberta.ca (login in as a guest, mathematics, grade 5, multiplication and division of
decimals. This first half of the video, or first 15 minutes, deals with just multiplication.