All over TikTok, “girl math” is being used to justify almost any purchase, from everyday things like $5 coffees to big-ticket items like concert tickets. While everyone’s version of girl math is different, it typically involves rationalizing your spending in often illogical ways. Say you buy your daily coffee with cash, rather than your credit card — it’s basically free, according to girl math. Or perhaps you make an expensive purchase, like a designer handbag — just use girl math to break the price down on a “cost per wear” basis so it doesn’t seem as pricey as it really...