Mark up can be either an amount or a percentage and usually applies to percent questions.

We usually see mark up as a % change in price of an item. For example:

"If a store buys an item for $15 and then marks it up by 20%, what is the sale price of the item?"

"If after purchasing an item for $n a store marks it up by 25% and then sells the item for $20, what's the value of n?"

try plugging in smart numbers because they usually yield nice answers and eliminate alot of the confusion rendering the once hard question pretty easy.