STRAND: Number
TOPIC:
Decimal numbers
Multiplying decimal numbers
by whole numbers and decimals
To multiply a decimal number by a whole number, or by another decimal number, first perform the multiplication, ignoring the
decimal point(s), as you would if both numbers were whole. To find the position of the decimal point, count the total number of
decimal places in the question and then put the decimal point in the answer so that it (the answer) has the same number of
decimal places.
Note that when setting out multiplication, it is not necessary to position the numbers so that the decimal points are in line.
WORKED Example
Multiply each of the following.
a 0.78 × 5
b 1.31 × 3.2
THINK
WRITE
a Multiply, ignoring the decimal point (that is,
multiply 78 by 5). Count the digits after the
decimal point; there are two (7 and 8). Put the
decimal point in the answer so that there are
two digits after it.
a
b Multiply, ignoring the decimal point (that is,
multiply 131 by 32). Count all digits after the
decimal points; there are 3 (two in the first
number and one in the second). Put the
decimal point in the answer so that it creates
three decimal places.
b × 1.31
× 3.2
× . 262
+ .3930
× 4.192
03.3748
× 03.3745
33.3930
Try these
Multiply each of the following.
1 0.79 × 6
2 0.65 × 5
3 1.79 × 7
4 0.73 × 13
5 0.96 × 16
6 1.85 × 21
7 0.62 × 0.5
8 0.03 × 1.7
9 0.9 × 3.5
10 0.4 × 1.2
11 0.55 × 5.2
12 0.04 × 9.1
 John Wiley & Sons Australia, Ltd 2001