One of social media’s most popular, and long-running trends, has been the “Girl” hashtag. TikTok users are showing off their #girldinner as they live their #hotgirlsummer, and brands such as Popeyes have jumped on the trend. The hashtags have expanded to include more adjectives, from #cleangirl to #strawberrygirl.

Now, in an economy in which price is factoring into merchandise haul videos and retailers are hard-pressed to tout their deals and discounts, the use of Girl Math—#girlMath—is on an upswing.

The trend began with videos in which women depict how they are spending their money, including how they justify and rationalize buying items (whether on sale or not on sale) and consider cost-per-wear metrics ahead of purchases.

More recent posts from marketers showcase value. “The girl math all checks out to us. If your shopping haul was all deals, does it even count as real money?” read a late August TikTok post from discount retail chain Five Below.

The #girlmath hashtag currently has more than 155 million views on TikTok. Many of the videos have been posts made by influencers and everyday consumers. And brands, recognizing the opportunity, have begun to jump in. Marketers such as Babylist, the baby registry brand, Ulta Beauty and Rihanna’s Fenty Beauty are among those doing girl math.

Lee Anne Grant, chief growth officer at Babylist, noted that the company is always looking at trends and trying to find a way to participate in cultural conversations.

“Our take on ‘girl math’ was to share some of the ‘parent math’ that comes directly from the experience of some of the parents on our team and is incredibly relatable to our customers,” Grant wrote in an email, noting that Babylist tries to put its own spin on trends to make them particularly relatable to its 1.2 million followers. “We create TikToks that share all the ups and downs of the pregnancy and new baby experience—which often means not taking ourselves too seriously,” Grant added.

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Ulta used #girlmath to promote its loyalty program as a way of driving down costs for customers.

Beauty brand NYX combined both #girldinner and #girlmath in one recent post.

The trends are taking hold at a time when social media, and TikTok in particular, is a key instigator of packaged goods buys, especially for Gen Z. A recent study from performance marketing agency Tinuiti found that social media contributes to more of such purchases than TV for the cohort.

But marketers need to get it right. The danger lies in offending customers by falling into a sexist trap that condescends to tell women they are bad at math. According to Cindy Gallop, a gender equality advocate, consultant and CEO of IfWeRanTheWorld, the way to do this is to be “original, creative and empathetic,” and acknowledge that #girlmath is a joke.

“The fatal thing for a brand to do would be to take this literally and talk down to consumers,” she wrote in an email. “I see too many brands fail to respect consumers, especially when those consumers are female.”