6.2 Solve general applications of percent

Translate and solve basic percent equations

We will solve percent equations by using the methods we used to solve equations with fractions or decimals. In the past, you may have solved percent problems by setting them up as proportions. That was the best method available when you did not have the tools of algebra. Now as a prealgebra student, you can translate word sentences into algebraic equations, and then solve the equations.

We'll look at a common application of percent—tips to a server at a restaurant—to see how to set up a basic percent application.

When Aolani and her friends ate dinner at a restaurant, the bill came to $\text{\$80}.$ They wanted to leave a $\text{20\%}$ tip. What amount would the tip be?

To solve this, we want to find what amount is $\text{20\%}$ of $\text{\$80}.$ The $\text{\$80}$ is called the base . The amount of the tip would be $0.20\left(80\right),$ or $\text{\$16}$ See [link] . To find the amount of the tip, we multiplied the percent by the base.

In the next examples, we will find the amount. We must be sure to change the given percent to a decimal when we translate the words into an equation.

What number is $\text{35\%}$ of $90?$

Solution

Translate into algebra. Let $n=$ the number.
Multiply.	$31.5$ is $\mathrm{35\%}$ of $90$

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$\text{125\%}$ of $28$ is what number?

Solution

Translate into algebra. Let $a=$ the number.
Multiply.	$\mathrm{125\%}$ of $28$ is $35$ .

Remember that a percent over $100$ is a number greater than $1.$ We found that $\text{125\%}$ of $28$ is $35,$ which is greater than $28.$

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In the next examples, we are asked to find the base.

Translate and solve: $36$ is $\text{75\%}$ of what number?

Solution

Translate. Let $b=$ the number.
Divide both sides by 0.75.
Simplify.

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$\text{6.5\%}$ of what number is $\text{\$1.17}?$

Solution

Translate. Let $b=$ the number.
Divide both sides by 0.065.
Simplify.

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In the next examples, we will solve for the percent.

What percent of $36$ is $9?$

Solution

Translate into algebra. Let $p=$ the percent.
Divide by 36.
Simplify.
Convert to decimal form.
Convert to percent.

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$144$ is what percent of $96?$

Solution

Translate into algebra. Let $p=$ the percent.
Divide by 96.
Simplify.
Convert to percent.

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Solve applications of percent

Many applications of percent occur in our daily lives, such as tips, sales tax, discount, and interest. To solve these applications we'll translate to a basic percent equation, just like those we solved in the previous examples in this section. Once you translate the sentence into a percent equation, you know how to solve it.