Mark Zegarelli - Basic Math & Pre-Algebra All-in-One For Dummies (+ Chapter Quizzes Online)

The Big Four operations are actually two pairs of inverse operations, which means the operations can undo each other.

Addition and subtraction: Subtraction undoes addition. For example, if you start with 3 and add 4, you get 7. Then, when you subtract 4, you undo the original addition and arrive back at 3:This idea of inverse operations makes a lot of sense when you look at the number line. On a number line, means start at 3, up 4. And means start at 7, down 4. So when you add 4 and then subtract 4, you end up back where you started.

Multiplication and division: Division undoes multiplication. For example, if you start with 6 and multiply by 2, you get 12. Then, when you divide by 2, you undo the original multiplication and arrive back at 6:

The commutative property of addition tells you that you can change the order of the numbers in an addition problem without changing the result, and the commutative property of multiplication says you can change the order of the numbers in a multiplication problem without changing the result. For example,

In contrast, subtraction and division are non-commutative operations. When you switch the order of the numbers, the result changes.

Here’s an example of how subtraction is non-commutative:

Subtraction is non-commutative, so if you have $6 and spend $4, the result is not the same as if you have $4 and spend $6. In the first case, you still have $2 left over. In the second case, you owe $2. In other words, switching the numbers turns the result into a negative number. (I discuss negative numbers in Chapter 4.)

And here’s an example of how division is non-commutative:

For example, when you have five dog biscuits to divide between two dogs, each dog gets two biscuits and you have one biscuit left over. But when you switch the numbers and try to divide two biscuits among five dogs, you don’t have enough biscuits to go around, so each dog gets none and you have two left over.

Through the commutative property and inverse operations, every equation has four alternative forms that contain the same information expressed in slightly different ways. For example, and are alternative forms of the same equation but tweaked using the commutative property. And is the inverse of . Finally, is the inverse of .

You can use alternative forms of equations to solve fill-in-the-blank problems. As long as you know two numbers in an equation, you can always find the remaining number. Just figure out a way to get the blank to the other side of the equals sign.

When the first number is missing in any problem, use the inverse to turn the problem around:

When the second number is missing in an addition or multiplication problem, use the commutative property and then the inverse:

When the second number is missing in a subtraction or division problem, just switch around the two values that are next to the equals sign (that is, the blank and the equals sign):

Q.What’s the inverse equation to

A. .In the equation , you start at 16 and subtract 9, which brings you to 7. The inverse equation undoes this process, so you start at 7 and add 9, which brings you back to 16:

Q.What’s the inverse equation to

A. In the equation , you start at 6 and multiply by 7, which brings you to 42. The inverse equation undoes this process, so you start at 42 and divide by 7, which brings you back to 6:

Q.Use inverse operations and the commutative property to find three alternative forms of the equation .

A. First, use inverse operations to change subtraction to addition: