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Matein Extra-Large Carry On Backpack, $24 (was $44)

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Why We Love This Deal

I remember asking my 7th-grade geometry teacher the dreaded question: “When will we ever use this?” The rehearsed answer? “For you, John? Never.” But for those who go on to become creatives, designers , and engineers, these early geometry classes are critical. It's the reason a product like this seemingly simple backpack that Amazon sells for $24 has become such a massive hit.

The Matein Extra-Large Carry On Backpack might sell for under $25, but it's become the site's No. 1 casual daypack for one reason: geometry.…