Solve Equations Using the Division and Multiplication Properties of Equality

You may have noticed that all of the equations we have solved so far have been of the form or . We were able to isolate the variable by adding or subtracting the constant term on the side of the equation with the variable. Now we will see how to solve equations that have a variable multiplied by a constant and so will require division to isolate the variable.

Let’s look at our puzzle again with the envelopes and counters in (Figure 1).

In the illustration there are two identical envelopes that contain the same number of counters. Remember, the left side of the workspace must equal the right side, but the counters on the left side are “hidden” in the envelopes. So how many counters are in each envelope?

How do we determine the number? We have to separate the counters on the right side into two groups of the same size to correspond with the two envelopes on the left side. The 6 counters divided into 2 equal groups gives 3 counters in each group (since ).

What equation models the situation shown in (Figure 2)? There are two envelopes, and each contains counters. Together, the two envelopes must contain a total of 6 counters.

If we divide both sides of the equation by 2, as we did with the envelopes and counters,
we get:

We found that each envelope contains 3 counters. Does this check? We know , so it works! Three counters in each of two envelopes does equal six!

This example leads to the Division Property of Equality.

When you divide or multiply both sides of an equation by the same quantity, you still have equality.

Let’s review how these properties of equality can be applied in order to solve equations. Remember, the goal is to ‘undo’ the operation on the variable. In the example below the variable is multiplied by , so we will divide both sides by to ‘undo’ the multiplication.

EXAMPLE 1

Solve: .

Solution

We use the Division Property of Equality to divide both sides by .

Divide both sides by 4 to undo the multiplication.
Simplify.
Check your answer. Let .

Since this is a true statement, is a solution to .

In the previous example, to ‘undo’ multiplication, we divided. How do you think we ‘undo’ division?

EXAMPLE 2

Solve: .

Solution

Here is divided by . We can multiply both sides by to isolate .

Multiply both sides by .
Simplify.
Check your answer. Let .

EXAMPLE 3

Solve: .

Solution

Remember is equivalent to .

Rewrite as .
Divide both sides by .
Check.
Substitute
Simplify.

We see that there are two other ways to solve .

We could multiply both sides by .

We could take the opposite of both sides.

TRY IT 3.1

Solve: .

Show answer

k = −8

TRY IT 3.2

Solve: .

Show answer

g = −3

EXAMPLE 4

Solve: .

Solution

Since the product of a number and its reciprocal is , our strategy will be to isolate by multiplying by the reciprocal of .

Multiply by the reciprocal of .
Reciprocals multiply to one.
Multiply.
Check your answer. Let

Notice that we could have divided both sides of the equation by to isolate . While this would work, multiplying by the reciprocal requires fewer steps.

TRY IT 4.1

Solve: .

Show answer

n = 35

TRY IT 4.2

Solve: .

Show answer

y = 18

Solve Equations That Need to be Simplified

Many equations start out more complicated than the ones we’ve just solved. First, we need to simplify both sides of the equation as much as possible

EXAMPLE 5

Solve: .

Solution

Start by combining like terms to simplify each side.

Combine like terms.
Divide both sides by 12 to isolate x.
Simplify.
Check your answer. Let

EXAMPLE 6

Solve: .

Solution

Simplify each side by combining like terms.

Simplify each side.
Divide both sides by 3 to isolate y.
Simplify.
Check your answer. Let

Notice that the variable ended up on the right side of the equal sign when we solved the equation. You may prefer to take one more step to write the solution with the variable on the left side of the equal sign.

EXAMPLE 7

Solve: .

Solution

Remember—always simplify each side first.

Distribute.
Simplify.
Divide both sides by -3 to isolate n.
Check your answer. Let .

Practice exercises

Exercises

Solve Equations Using the Division and Multiplication Properties of Equality

In the following exercises, solve each equation for the variable using the Division Property of Equality and check the solution.

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In the following exercises, solve each equation for the variable using the Multiplication Property of Equality and check the solution.

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Exercises

Solve Equations That Need to be Simplified

In the following exercises, solve the equation.

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35.	36.

Exercises

Everyday Math

37. Teaching Connie’s kindergarten class has children. She wants them to get into equal groups. Find the number of children in each group, , by solving the equation .	38. Balloons Ramona bought balloons for a party. She wants to make equal bunches. Find the number of balloons in each bunch, , by solving the equation .
39. Unit price Nishant paid for a pack of juice bottles. Find the price of each bottle, , by solving the equation .	40. Ticket price Daria paid for children’s tickets at the ice skating rink. Find the price of each ticket, , by solving the equation .
41. Fabric The drill team used yards of fabric to make flags for one-third of the members. Find how much fabric, , they would need to make flags for the whole team by solving the equation .	42. Fuel economy Tania’s SUV gets half as many miles per gallon (mpg) as her husband’s hybrid car. The SUV gets . Find the miles per gallons, , of the hybrid car, by solving the equation .

Writing Exercises

43. Emiliano thinks is the solution to the equation . Explain why he is wrong.

44. Frida started to solve the equation by adding to both sides. Explain why Frida’s method will result in the correct solution.

 

Answers