In this video, we will be learning how to find a percent of a number using proportions. After you finish this lesson, view all of our Pre-Algebra and Algebra lessons and practice problems.

In math, “of” means multiply

Percent Formula: \frac{Percent}{100}=\frac{Part (is)}{Whole(of)}

Example of Finding the Percent of a Number

What is 8% of 40?

\frac{8}{100}=\frac{x}{40}\leftarrow First we cross-multiply

\frac{320}{100}=\frac{100x}{100}\leftarrow Then we divide by 100 on both sides to isolate x

3.2=x

Example 1

What is 17\% of 30?

First, we cross-multiply

\dfrac{17}{100}=\dfrac{x}{30}

Then, we divide by 100 on both sides to isolate x

\dfrac{510}{100}=\dfrac{100x}{100}

5.1=x

The answer is 5.1

Example 2

What percent of  50 is 4.8?

We have the “of” and the “is”. Our formula should look like this:

\dfrac{x}{100}=\dfrac{4.8}{50}

Let’s cross multiply

x\times50 is 50x

4.8 \times 100 is 480

We have:

50x=480

Now, let’s divide both sides by 50

\dfrac{50x}{50}=\dfrac{480}{50}

x=9.6

Therefore, the answer is 9.6\%

Video-Lesson Transcript

In this video, we will be learning how to find a percent of a number using proportions.

In math, ‘of’ means ‘multiply’.

So if you’re asked 8\% of 40, it means 8\% \times 40.

We can’t get the answer right away.

We have to change 8\% into a decimal.

So we have to move the decimal two spaces from the right to left.

8\% will become 0.08 then we can multiply

0.08 \times 40 = 3.2

Another way to find the percent of a number is the percent formula.

Percent formula is \dfrac{Percent}{100} = \dfrac{Part (is)}{Whole (of)}

Let’s answer this question:

What is 8\% of 40?

Let’s use the formula

\dfrac{8}{100} = \dfrac{x}{40}

Let’s cross-multiply

8 \times 40 is 320

x \times 100 is 100x

Now we have 320 = 100x

To isolate x, let’s do the inverse operation.

We just have to divide both sides by 100

And we get 3.2 = x

Percent of a Number

Which is the same answer using a different method.

To sum up, you can multiply or use the percent formula to get the answer if the missing value is the “is” or the part.

But if the missing value is the percent or the whole, you have to use the percent formula.

To give an example, let’s answer this question:

What percent of 40 is 3.2?

Here, we’re looking for the percent. We have the “of” and the “is”.

The formula is \dfrac{Percent}{100} = \dfrac{Part (is)}{Whole (of)}

Since we’re missing the percent, our formula should look like this

\dfrac{xt}{100} = \dfrac{3.2}{40}

Then let’s cross multiply

x \times 40 is 40x

3.2 \times 100 is 320

Now, let’s do the inverse operation of multiplication which is division.

So, let’s divide both sides by 40

\dfrac{40x}{40} = \dfrac{320}{40}

The answer is x = 8

So the answer to what percent of 40 is 3.2?

The answer is 8\%