If you need to justify buying a luxury item, try using “girl math” to make those calculations!

X, formerly Twitter, user @iamsashakae says if you pay in cash, the item is “free”, and if Sephora is having a sale, you’re losing money if you don’t buy what you “need.”

Other girl math tricks include rounding figures down and dividing the item’s price by how many times you will use or wear that item. For example, if you wear a $300 dress 20 times, you’re really “only” paying $15…according to girl math.

Some girl math actually makes sense, according to economist Brad Olsen.

“Everyone’s now thinking like an economist…’Where’s the economies of scale? How do I get the most bang for [the] buck here,’ and I like that thinking,” Olsen said. (Mashable)