Whether you took the SAT, GRE, or another standardized test decades ago, or just this year, perhaps you still remember a few pesky questions that tripped you up and sabotaged your final score. Take __this math question__—one of those that seemingly everyone gets wrong.

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It’s a line with two intersecting curves: one reaching both ends of the line without crossing it, the other crossing the line before it reaches the endpoint on the other side. If you’re in a mellow state of mind, it may remind you of a yin-yang.

The question: What do you know about the length of the two curves? Is one longer, are they the same length, or is it impossible to say?

The diagram looks fairly simple, but answering correctly requires remembering some basic facts about circles, as mathematician Presh Talwalkar shows in one of his *Mind Your Decision *videos.

Start by assigning a value to the line; let’s say it’s 10. Using the formula for the circumference of a circle, we can figure out that the curve that doesn’t cross the line is 5 times pi. So far, so easy.

Figuring out the length of the other curve is a little trickier, because it intersects the line. But if you’ve already said the line is 10, the line segment under one side of the curve is *x, *while the other is 10-*x.*

Substitute those numbers back into the formula for a circle’s circumference. It turns out that, in fact, the two curves are the same length. How’s that for math magic?

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From: Men's Health US