The word "percent" means "out of 100." Note 6 For example, 49% means 49 out of 100. Percents can also be expressed as fractions or decimals, since they too can be used to imply some part of a whole. So 49% can also be written as 49/100 or 0.49.
A percent implies a ratio: It is some part "per 100." Ratios enable us to set up a relationship between two numbers. For example, in a water molecule, there is always one oxygen atom for every two hydrogen atoms, which means that the ratio of oxygen to hydrogen is 1:2. In a percent, the second number in the ratio is always 100. Such ratios always express a number of parts per 100 parts.
You can approach any kind of percent problem if you think of it as a proportion that equates two ratios: a data ratio and a percent ratio. In other words:
Since the percent whole is always 100, we can substitute 100 for "percent whole" in this formula:
Notice that there are three different unknowns in this equation. If you know any two of them, you can easily find the third.
For example, if you want to know how much 30% of $150 is, you'd write the proportion as follows:
From here, you can easily calculate the value you're looking for, which in this case is $45.