How to express one quantity as a percentage of another?

There are several ways to do this which make it particularly interesting. I would advise trying to avoid doing it by eye, this leaves you open to many clumsy mistakes. My preferred method is using equivalent fractions. Scale the fraction up on the denominator until you have your required /100 in order to read it as a percentage. Remember, it is extremely important to obey the rules of equivalent fractions: whatever you do to the denonimator you MUST do to the numerator.