Math comes naturally to some, but even simple equations remain baffling brainteasers to others. These math equations went viral for being much more complicated than they seemed — or so simple that people got tripped up overthinking them. Here are 9 math problems that confused people across the internet.

1. Spotted on The Daily Mail, the question was originally created by Go Tumble and shared on Wikr before taking off on Facebook and going viral. There are two correct ways to solve it. The first way to find the solution is to add the equation, then combine the sum with that of the previous equation. The second solution involves multiplying the second number of the equation by the number you are adding to it. The correct answer could either be 40 or 96.

2. This Common Core math quiz caused a firestorm on Reddit. The first question asks the student to calculate 5 x 3 using repeated addition. The student wrote 5 + 5 + 5 = 15, and was marked wrong, with the teacher writing in the “correct” solution of 3 + 3 + 3 + 3 + 3 = 15. The second question prompts the student to calculate 4 x 6 using an array. The student drew an array with six rows and four columns, getting the answer that 4 x 6 = 24. The teacher marked the question wrong again and drew in a nearly identical array of four rows and six columns. “The idea that a student should be punished for recognizing and applying the fundamental truth of commutative multiplication in service of drilling in a completely arbitrary convention that they can easily learn when they need it 10 years later strikes me as borderline insane,” Andy Kiersz of Business Insider wrote.

3. This math problem from Singapore went viral in the US. Kenneth Kong, a television host in Singapore, shared a photo of this 5th grade-level math question in a since-deleted Facebook post, which was shared nearly 6,000 times. In the logic puzzle, Cheryl gives her friends Albert and Bernard different clues as to when her birthday is out of a selection of dates. She tells Albert only the day and Bernard only the month of her birthday. By making a table of the dates and using the process of elimination, one can determine that Cheryl’s birthday is July 16. It was later revealed that this problem wasn’t a regular test question used in Singapore classrooms. It was actually used in a contest as part of the Singapore and Asian Schools Math Olympiad (SASMO).

4. This second grade math question stumped kids and their parents. A UK mom tweeted this math problem in a since-deleted tweet saying “Have you seen this one? Year 2!!” It was then picked up by a Facebook page called Parents Against Primary Testing and media outlets like The Huffington Post. Calculating the answer is simpler than it seems: 19 people getting off the train can be represented by -19, and 17 people getting on the train as +17. -19 + 17 = 2, meaning that there was a net loss of two people. If there are 63 people on the train now, that means there were 65 to begin with. That said, many are convinced the answer is 46.

5. This seemingly simple math problem racked up over five million views on YouTube. The correct way to solve this problem is to use the modern interpretation of the order of operations, also known as PEMDAS or BODMAS:

Parentheses/Brackets

Exponents/Orders

Multiplication-Division

Addition-Subtraction

If same precedence, left to right

The correct answer is 9, but controversy ensued because the historical order of operations from before 1917 differs slightly. With that version of the rules, which is still taught in many schools, the answer would be 1.

6. This question doesn’t actually involve math at all. The Guardian pointed out the simple solution: turn the picture upside down and you’ll see that the numbers are in numerical order from 86 to 91.

7. This word problem is a trick question. Nothing is actually missing here — it’s just deliberately confusing wording. It all adds up if you look at the total, not the debt owed. Twitter user Mat Whitehead laid it out in a table to show that there’s not a missing $1 after all.

8. This math question from Vietnam isn’t that difficult, but extremely time consuming. The challenge: use each digit 1-9 only once to fill in the snake and make the equation equal 66 (colons are division signs). According to VNEXPRESS, this puzzle is meant for third graders. There’s no trick or complicated math necessary — finding the correct configuration of numbers comes down to trial and error and process of elimination. Here’s a tip: it’s easier if you rewrite the snake as an equation and follow the order of operations.

9. Allegedly, only one out of 10 people could ace his math quiz without a calculator. No calculator? No problem. The easiest way to go about solving this without a calculator is to round the numbers up or down to multiples of five, estimate the answer, and choose the option closest to your estimate.