We can use a double number line diagram to show information about percent increase and percent decrease:

Double number line, 8 evenly spaced tick marks. Top line, cereal, grams. Beginning at first tick mark, labels: 0, 100, 200, 300, 400, 500, 600, 700. Bottom line, beginning at first tick mark, labels: 0 percent, 20 percent, 40 percent, 60 percent, 80 percent, 100 percent, 120 percent, 140 percent.

The initial amount of cereal is 500 grams, which is lined up with 100% in the diagram. We can find a 20% increase by adding 20% of 500:

If the initial amount of 500 grams is decreased by 40%, we can find how much cereal there is by subtracting 40% of the 500 grams:

To solve percentage problems, we need to be clear about what corresponds to 100%. For example, suppose there are 20 students in a class, and we know this is an increase of 25% from last year. In this case, the number of students in the class last year corresponds to 100%. So the initial amount (100%) is unknown and the final amount (125%) is 20 students.

Double number line, 8 evenly spaced tick marks. Top line, number of students. Beginning at first tick mark, labels: 0, 4, 8, 12, 16, 20, 24, 28. Bottom line, beginning at first tick mark, labels: 0 percent, 25 percent, 50 percent, 75 percent, 100 percent, 125 percent, 150 percent, 175 percent. 16 and 100 percent are circled.

Looking at the double number line, if 20 students is a 25% increase from the previous year, then there were 16 students in the class last year.