A percentage is a way of expressing a number as a fraction of 100. The word comes from the Latin "per centum," meaning "by the hundred." Percentages are used everywhere in daily life, from sales tax and discounts to interest rates and statistics.

There are three fundamental percentage calculations. Finding X% of Y uses the formula X/100 x Y. Finding what percent X is of Y uses X/Y x 100. And finding the whole when you know the part and percentage uses Part / (Percentage/100). Each of these is a rearrangement of the same basic relationship.

For example, 15% of 200 is 30 (found by multiplying 200 by 0.15). Conversely, 30 is 15% of 200 (found by dividing 30 by 200 and multiplying by 100). And if you know that 30 is 15% of some number, you can find that number is 200 by dividing 30 by 0.15. Mastering these three calculations covers virtually every percentage problem you will encounter.