This is a complete lesson for learning how to calculate percents. Percents are common in the real world outside of math class, but may be hard for students to understand. However, they are best understood as a ratio shown as a number over 100, for example 27/100; or 27 percent. This lesson presents two different ways to calculate a percent from a ratio. One way is to set up a proportion with the original ratio on one side, and the percent on the other, like this: 4/25 = X/100. Cross-multiply