Description: In this example, you working with us to find the number that is expressed as a given percentage. We'll create a simple algebraic equation to solve! Created by Sal Khan and Monterey Institute for Technology and Education. So let's just call that unknown number x. Now, 15% mathematically, you can deal directly with percentages, but it's much easier if it's written as a decimal. So we could rewrite this as 0.15 times some unknown number, times x, is equal to 78.*Teacher Advisor is 100% free.