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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