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3.4: Solve Equations Involving Inverse Properties of Subtraction and Division

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Did you ever solve a dilemma about wrapping paper? Take a look at this one.

Brandon and Felicia sold rolls of wrapping paper for a school fundraiser. Brandon sold 3 less than half the number of rolls that Felicia sold. Brandon sold a total of 9 rolls of wrapping paper.

Write an algebraic equation to represent f , the number of rolls of wrapping paper that Felicia sold. Then, find the number of rolls of wrapping paper that Felicia sold.

Do you know how to solve this dilemma? Notice that there will be two parts to your answer. Pay attention to this Concept and you will know how to figure out this problem.

Guidance

You are going to learn how to solve two-step equations with subtraction and division. Let’s begin.

To solve a two-step equation, we will need to use more than one inverse operation. Let's take a look at how to solve a two-step equation now. When we perform inverse operations to find the value of a variable, we work to get the variable alone on one side of the equals. This is called isolating the variable. It is one strategy for solving equations. You can use isolating the variable whether you are solving one-step or two-step equations.

Solve for z : \frac{z}{6} - 7 = 3 .

Notice that there are two terms on the left side of the equation, \frac{z}{6} and 7. Our first step should be to use inverse operations to get the term that includes a variable, \frac{z}{6} , by itself on one side of the equal (=) sign.

In the equation, 7 is subtracted from \frac{z}{6} . So, we can use the inverse of subtraction—addition. We can add 7 to both sides of the equation, like this:

\frac{z}{6} - 7 & = 3\\\frac{z}{6} - 7 + 7 & = 3 + 7\\\frac{z}{6} + (-7 + 7) & = 10\\\frac{z}{6} + 0 & = 10\\\frac{z}{6} &= 10

Now, the term that includes a variable, \frac{z}{6} , is by itself on one side of the equation.

We can now use inverse operations to get the z by itself. Since \frac{z}{6} means z \div 6 , we can use the inverse of division—multiplication. We can multiply both sides of the equation by 6, like this:

\frac{z}{6} &= 10\\\frac{z}{6} \times 6 & = 10 \times 6\\\frac{z}{\bcancel{6}} \times \frac{\bcancel{6}}{1} & = 60\\\frac{z}{1} & = 60\\z & = 60

The value of z is 60.

Let’s review our steps to solving this two-step equation.

Take a few minutes to write these steps in your notebook.

Example A

\frac{x}{3} - 8 = 9

Solution:  x = 51

Example B

\frac{y}{7} - 2 = 13

Solution:  y = 105

Example C

\frac{a}{7} - 2 = 12

Solution:  a = 98

Now let's go back to the dilemma at the beginning of the Concept.

Consider part a first.

Use a number, an operation sign, a variable, or an equal sign to represent each part of that problem. Since Brandon sold 9 rolls of wrapping paper, represent the number of rolls Brandon sold as 9. Use the key words from the chart to help you translate the rest of the problem into an equation. For example, you can translate “half the number of rolls...Felicia sold” as \frac{f}{2} .

& \underline{Brandon} \ \underline{sold} \ \underline{3} \ \underline{less \ than} \ \underline{half \ the \ number \ldots \ Felicia \ sold.}\\& \quad \ \downarrow \qquad \ \downarrow \quad \ \searrow \qquad \downarrow \qquad \qquad \quad \swarrow\\& \quad \ \downarrow \qquad \ \downarrow \quad \ \searrow \qquad \downarrow \qquad \qquad \quad \swarrow\\& \quad \ \downarrow \qquad \ \downarrow \quad \ \swarrow \qquad \downarrow \qquad \qquad \quad \searrow\\& \quad \ \ 9 \qquad = \ \ \frac{f}{2} \qquad - \qquad \qquad \quad 3

So, this equation, 9 = \frac{f}{2} - 3 , represents f , the number of rolls of wrapping paper that Felicia sold.

Next, consider part b .

Solve the equation for f to find the number of rolls that Felicia sold.

Our first step should be to use inverse operations to get the term that includes a variable, \frac{f}{2} , by itself on one side of the equal (=) sign. In the equation, 3 is subtracted from \frac{f}{2} . So, we can use the inverse of subtraction and add 3 to both sides of the equation, like this:

9 &= \frac{f}{2}-3\\9 + 3 & = \frac{f}{2} - 3 + 3\\12 & = \frac{f}{2} + (-3 + 3)\\12 & = \frac{f}{2} + 0\\12 & = \frac{f}{2}

Now, the term that includes a variable, \frac{f}{2} , is by itself on one side of the equation.

We can now use inverse operations to get the f by itself. Since \frac{f}{2} means f \div 2 , we can use the inverse of division and multiply both sides of the equation by 2, like this:

12 &= \frac{f}{2}\\12 \times 2 & = \frac{f}{2} \times 2\\24 & = \frac{f}{\bcancel{2}} \times \frac{\bcancel{2}}{1}\\24 & = \frac{f}{1}\\24 & = f

The value of f is 24, so Felicia sold 24 rolls of wrapping paper for the fundraiser.

Vocabulary

Equation
a mathematical statement with an equal sign where the quantity on one side of the equation is equal to the quantity on the other side.
Variable
a letter used to represent an unknown quantity.
Algebraic Equation
An equation with at least one variable in it.
One-Step Equation
An algebraic equation with one operation in it.
Two-Step Equation
An algebraic equation with two operations in it.

Guided Practice

Here is one for you to try on your own.

\frac{x}{6} - 9 = 8

Solution

To solve this problem, first we have to add nine to both sides of the equation.

\frac{x}{6} - 9 + 9 & = 8 + 9 \\\frac{x}{6} & = 17

Next, multiply 6 times 17.

x = 102

This is the answer.

Video Review

Solving Two-Step Equations

Practice

Directions: Solve each two-step equation that has division and subtraction in it.

  1. \frac{x}{5} - 4 = 8
  2. \frac{y}{6} - 3 = 8
  3. \frac{x}{7} - 7 = 10
  4. \frac{x}{8} - 4 = 12
  5. \frac{y}{7} - 5 = 11
  6. \frac{x}{4} - 10 = 12
  7. \frac{y}{4} - 8 = 2
  8. \frac{x}{3} - 12 = 9
  9. \frac{a}{5} - 3 = 11
  10. \frac{b}{4} - 1 = 15
  11. \frac{x}{2} - 8 = 4
  12. \frac{a}{7} - 4 = 9
  13. \frac{b}{4} - 7 = 3
  14. \frac{x}{8} - 1 = 12
  15. \frac{y}{6} - 8 = 5
  16. \frac{x}{2} - 15 = 12

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Dec 19, 2012

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Aug 21, 2014
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