How do I explain how to find one quantity as a percentage of another

You usually explain a process by describing how it is done. The ratio of two quantities is expressed as a percentage by multiplying that ratio by 100%.

Example

A as a percentage of B is (A/B)×100%.

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You will often see a description of finding a percentage as involving multiplication by 100. What multiplication by 100 does is give you a quantity 100 times as large. Generally, that will not be the number you are looking for.

For example, one out of four as a percentage is not (1/4)·100 = 25. 25 is two tens plus 5, a number way larger than 1/4. Rather, it is (1/4)·100% = 25%.

"Percent" and "hundredths" are interchangeable. The percent symbol (%) is a shorthand way to write /100, which is what it means. Consider the above example:

25% = 25/100 = 1/4 . . . . these are all the same value, written differently

In decimal, ...

25% = 25/100 = 0.25 . . . . these are all the same value, written differently