If you want to calculate a percentage of a number in Excel, simply multiply the percentage value by the number that you want the percentage of.
For example, if you want to calculate 20% of 500, multiply 20% by 500.
I.e. type the following formula into any Excel cell:
 which gives the result 100.
Note that the % operator tells Excel to divide the preceding number by 100. Therefore, the value 20% in the above calculation is evaluated as 0.2.
The following spreadsheet shows various percentage calculations for different numbers.
Formulas:

Results:

The following spreadsheet shows the calculation of sales tax at 22.5%, of an invoice total.
Formulas:

Results:

For further examples of Calculating Percentages of Numbers in Excel, see the Microsoft Office website.
Or, for an overview of different types of percentage calculation, see the Percentages In Excel page.