Know what a percentage is. A percentage is a way to express a number as a part of a whole. To calculate a percentage, we look at the whole as equal to 100%. For example, say you have 10 apples (=100%). If you eat 2 apples, then you have eaten 2/10 × 100% = 20% of your apples and you are left with 80% of your original apples. Determine the value of the whole. For instance, let's say we have a jar containing 1199 red marbles and 485 blue marbles, making it 1684 marbles in total. In this case, 1684 makes up a whole jar of marbles, and will be set equal to 100%. Find the value that you want to turn into a percentage. Let's say we want to find out the percentage of the jar that is taken up by the 485 blue marbles. Put the two values into a fraction.
In our example, we need to find out what percent 485 (number of blue marbles) is of 1684 (total number of marbles). Therefore the fraction in this case is 485/1684. Convert the fraction into a decimal. To turn 485/1684 into a decimal, divide 485 by 1684. This comes to 0.288. Convert the decimal into a percent. Multiply the result obtained in the step above by 100. For this example, 0.288 multiplied by 100 equals 28.8 or 28.8%. Know when you need to work backwards. Sometimes you're given the percentage of an amount and need to know the numerical value of the percent. Examples include calculating taxes, tips, and loan interest.
