The relationship @$\begin{align*}\text{percent} = \frac{\text{part}}{\text{whole}}\end{align*}@$ can be used to solve for a percent using the part and the whole. By multiplying the whole by the other side you get the following equation:@$$\begin{align*}\text{percent} \cdot \text{whole}= \text{part}\end{align*}@$$

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@$$\begin{align*}\text{Percent} \cdot \text{Whole}& = \text{Part} \\ \frac{20}{100} \cdot 90 & = \text{Part} \\ 18 & = \text{Part}\end{align*}@$$

Now, cross-multiply to find the cross-product. To cross-multiply, multiply the numerator of one fraction with the denominator of the other fraction. The products should be equal.

Finally, simplify the equation to solve for @$\begin{align*}x\end{align*}@$. Multiply 20 by 90 and divide both sides by 100.

Next, write a proportion to find the equivalent fraction of 20% of 90. The second fraction is the unknown quantity, represented by @$\begin{align*}x\end{align*}@$, over the whole, 90.

A percent is a ratio that describes a quantity out of 100. A percent is also a part of a whole. Finding the percent of a number is the part of a number that is equivalent to the percent.