There are three basic types of percentage problems:

MISSING PART:

Finding what quantity is a given percentage of another

There are 60 students in the class and 20% play football. How many students play football?

MISSING PERCENT:

Finding what percentage one quantity is of another

There are 60 students in the class and 12 play football. What percentage of the students play football?

MISSING WHOLE:

Finding the whole quantity (ie the 100%), given what percent a certain quantity is.

There are some students in a class and 12 play football. If 20% of the students in the class play football, how many students are there in the class?

PERCENT WHOLE - The percent whole is ALWAYS 100%.

The relationship between the part and the whole is the same as the relationship of a percent part to 100%.

As you can see in this image, the proportion between the part and the whole is the same as the proportion between the percent part and the percent whole.

A proportion occurs when two ratios are equal to each other. The proportion below can be stated as: “a is to b as c is to d.”

students are there in the class?

a is to b as c is to d.”http://www.colin.edu/gedonline/ILP/ILP%202-Solving%20Percent%20Problems.pdf