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3Multiplication
Suppose you go into a store and buy seven articles for $1.00 each. The total cost is
$7.00; you count $1.00 seven times. What if the articles cost $3.00 apiece? In that case,
to find the total, you must count $3.00 seven times. This type of problem leads to the
next step in calculating: multiplication.
IT’S A SHORTCUT
Multiplication is a shortcut for repeated addition. At one time, children learning arithmetic
memorized huge multiplication tables without knowing why they’d ever need
such facts. If you memorized, say, 7 times 3 is 21 (written 7 3 21), you could do
calculations more quickly than if you had to go to a table and look up every multiplication
fact. But not many people could tell you the reason why 7 3 21! Figure 3-1
shows why this is true: you must add up seven “threes” to get 21.
You won’t regret it if you memorize multiplication facts up through 9 9 81, as
shown in Table 3-1. These facts will be a great convenience if you know them “by
heart.” Then, along with the material you will learn in the rest of this chapter, when you
need to multiply two numbers together and you don’t happen to have a calculator
handy, you will be able to do it without any trouble.
MAKING A TABLE
The multiplication table was one of humankind’s earliest computers: a ready way of
getting answers without having to add things over and over. To understand it, you can
set up a multiplication table for yourself, as shown in Table 3-1. Start with the numbers
along the top and down the left side. Now, count in twos, and write the results in
the next column, under “2.” Each next figure down the column is 2 more than the one
above it. Now, do the same thing with the “3” column, adding 3 for each next figure
down the column. Continue until you have done the “9” column.
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