Basic Math and Pre-Algebra For Dummies (9 page)

Chapter 3

In This Chapter

Reviewing addition

Understanding subtraction

Viewing multiplication as a fast way to do repeated addition

Getting clear on division

When most folks think of math, the first thing that comes to mind is four little (or not-so-little) words: addition, subtraction, multiplication, and division. I call these operations the
Big Four
all through the book.

In this chapter, I introduce you (or reintroduce you) to these little gems. Although I assume you're already familiar with the Big Four, this chapter reviews these operations, taking you from what you may have missed to what you need to succeed as you move onward and upward in math.

Addition is the first operation you find out about, and it's almost everybody's favorite. It's simple, friendly, and straightforward. No matter how much you worry about math, you've probably never lost a minute of sleep over addition. Addition is all about bringing things together, which is a positive goal. For example, suppose you and I are standing in line to buy tickets for a movie. I have $25 and you have only $5. I could lord it over you and make you feel crummy that I can go to the movies and you can't. Or instead, you and
I can join forces, adding together my $25 and your $5 to make $30. Now, not only can we both see the movie, but we may even be able to buy some popcorn, too.

Addition uses only one sign — the plus sign (+): Your equation may read 2 + 3 = 5, or 12 + 2 = 14, or 27 + 44 = 71, but the plus sign always means the same thing.

 When you add two numbers together, those two numbers are called
addends
, and the result is called the
sum
. So in the first example, the addends are 2 and 3, and the sum is 5.

When you want to add larger numbers, stack them on top of each other so that the ones digits line up in a column, the tens digits line up in another column, and so on. (Chapter
2
has the scoop on digits and place value.) Then add column by column, starting from the ones column on the right. Not surprisingly, this method is called
column addition
. Here's how you add 55 + 31 + 12. First add the ones column:

Next, move to the tens column:

This problem shows you that 55 + 31 + 12 = 98.

Sometimes when you're adding a column, the sum is a two-digit number. In that case, you need to write down the ones digit of that number and carry the tens digit over to the next column to the left — that is, write this digit above
the column so you can add it with the rest of the numbers in that column. For example, suppose you want to add 376 + 49 + 18. In the ones column, 6 + 9 + 8 = 23, so write down the 3 and carry the 2 over to the top of the tens column:

Now continue by adding the tens column. In this column, 2 + 7 + 4 + 1 = 14, so write down the 4 and carry the 1 over to the top of the hundreds column:

Continue adding in the hundreds column:

This problem shows you that 376 + 49 + 18 = 443.

Subtraction is usually the second operation you discover, and it's not much harder than addition. Still, there's something negative about subtraction — it's all about who has more and who has less. Suppose you and I have been running on treadmills at the gym. I'm happy because I ran 3 miles, but then you start bragging that you ran 10 miles. You subtract and tell me that I should be very impressed that you ran 7 miles farther than I did. (But with an attitude like that, don't be surprised if you come back from the showers to find your running shoes filled with liquid soap!)

As with addition, subtraction has only one sign: the minus sign (−). You end up with equations such as 4 − 1 = 3, and 14 − 13 = 1, and 93 − 74 = 19.

 When you subtract one number from another, the result is called the
difference
. This term makes sense when you think about it: When you subtract, you find the difference between a higher number and a lower one.