Suppose there are 20 questions on a multiple-choice test. if 25% of the answers are choice b, how many of the answers are not choice b?

This is one method: Part=Percent • Whole
Your part is x since you don't know it yet. Your percent is 25%. Your whole is 20. However, you have to turn 25% into a decimal, which would be .25.
Your new equation is now x=.25•20. When you multiply .25 by 20, you should get 5. Therefore, 5 is how many questions ARE B. However, you need to find the questions that aren't B. This is why you would have to subtract 5 from 20, giving you 15 questions that are NOT B. That would be your final answer.

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An easier way is to divide you whole by 100 and multiply it by the percentage. 20 divided by 100 is .2. Then you multiply it by 25, giving you 5.