The question he poses is a simple one, albeit deceptively so: What fraction is shaded?
Though the main image is a simple square, that it’s broken down into four uneven “pieces” is what seems to be giving people the most trouble in figuring out the area of the pink shaded triangle.
Fortunately, hundreds of problem solvers were game to give the problem a try, and share how they came up with their answers with the Twitterverse—some of whom went full on Good Will Hunting on the problem:
Look closely at the solutions above and you’ll notice that they all come up with “1/3” as the answer, which is correct (even if they came to it in slightly different ways).
According to Business Insider reporter Andy Kiersz, the answer lies in figuring out the height of the pink triangle. “The key trick is that the little triangle up top is similar to the pink triangle, which means that the little triangle is just a smaller version of the pink triangle,” Kiersz wrote.
"A property of similar triangles is that the ratio of the triangles' heights will be the same as the ratio of their bases," Kiersz continues. "Since the pink triangle's base is twice the little triangle's base, its height is also twice the little triangle's height. But we know that the little triangle's height plus the pink triangle's height is 1, so that means the pink triangle's height is 2/3. Plug that on in and we get our area = 1/2 x base x height = 1/2 x 1 x 2/3 = 1/3."
(Southall confirmed that 1/3 is indeed the correct answer.)
[h/t: Business Insider]