Get out your thinking caps, and get ready to possibly lose some of your grown-up pride.

Some of the things you learn in elementary school, like arithmetic, basic science, and the fact that chocolate milk is always a good choice, are bits of knowledge that will serve you well throughout your entire life. Quite often, though, the bigger benefit of primary education is learning how to function and communicate as a member of society, which includes paying thorough attention and following necessary directions.

Driving home those last two concepts seems to be the point of a worksheet recently brought home by the elementary school-age child of Yoko Sanri (who’s coincidentally the creator of manga/anime B Gata H Kei).

On the paper are a series of numbers, with two numerals being the input for some mathematical function that produces the next number in the diagonally descending line. However, one of the numbers in that line has been replaced by a question mark, and the students were asked what number should go in that space. Take a moment and see if you can figure it out.

The first step in the chain uses 72 and 99 to produce 27, which is the difference between the two inputs. Next up, 27 and 45 somehow combine to form 18, which is again the difference between the two. 18 and 39 become 21, and at this point it looks like we’ve cracked the code, so the missing number, which is somehow a function of 21 and 36, should be 15, right? This even checks out at the next step, since the difference between 15 and 28 would be 13.

But 15 is the wrong answer.

Like we said at the beginning, one of the responsibilities elementary school teachers are tasked with is ensuring that kids learn to be thorough. So while the number in the diagonal line is indeed equal to the difference of the two inputs for the first five times, at the very bottom the inputs are 13 and 21, but the result isn’t 8. It’s 7, meaning what we’ve been looking at the whole time isn’t actually the difference between the inputs.

So what’s going on here? Well, let’s start by revealing the correct answer for the mystery number, which is…

…12!

What’s actually happening isn’t subtraction, but addition, although in an unexpected way. Let’s take another look at the first step, where 72 and 99 become 27. If you take each of the four digits in the inputs, 7, 2, 9, and 9, then add them all together, you’ll get 27, and it turns out this is what’s really being done in each step.

So when we’re one step before the mystery number, and the inputs are 36 and 21, we should break those into 3, 6, 2, and 1, which gives us 12. From there, inputting 12 and 28 gives us 13, and finally 13 and 21 become 7.

Sanri herself says that on her first attempt, she quickly came to the mistaken conclusion that the answer was 15, and it was only after she started patting herself on the back that she realized she wasn’t right and plenty of other Twitter users say they fell into the same trap. No word on whether or not Sanri’s kid got the correct answer, but even if he didn’t, he can take some solace in knowing that this math puzzle for elementary school students fooled plenty of adults.

Source: IT Media, Twitter/@sanriyoko