This Maths Puzzle Will Stump You For Eternity — or, Ya Know, Till You Look at the Answer

When I first came across this on Instagram, my train of thought went a little something like this: "Hmm, 4 + 3 = 7, so maybe I can just also add 3 to 7, but oh, wait, that equals 10 and not 13, so WTF is the secret here?! How do all of these numbers relate to each other, and more importantly, what's the darn answer?" I unfortunately didn't figure it out on my own and needed the trusty solution video below to clue me in. Can you figure this one out without taking a look at the answer?