May 5, 2019 Updated: May 5, 2019

A puzzle has gone viral on social media sites, and it has stumped many.

The blog Centauro says the puzzle is used in Hong Kong, and it appears on tests for children entering the first grade.

“This was one of the questions that children of the age of 6 faced as part of their entry exam to a primary school in Hong Kong, since this was published on the Chinese social media network Sina Weibo (Chinese twitter), it has been touted as one of the most popular things to come out of China since the invention of fireworks,” it says.

Students have less than 20 seconds to solve it.

You can find the answer below.

Answer: The trick with the puzzle is to flip the image upside down, therefore the answer is 87.

## Another ‘Viral’ Problem

At first glance, it looks easy. But it’s stumped many over the years, starting in Japan.

The problem went viral in Japan after a recent study found only 60 percent of people between the ages of 21 and 29 could get the correct answer–down from 90 percent in the 1980s.

Despite appearing simple, Presh Talwalker of Mind Your Decisions noted what had fooled most people.

“You should write an expression that groups one third as one group,” he says in the video.

“Three divided by one third is equal to nine, and now we have nine minus nine, plus one,” he added

The solution harkens back to an acronym some likely learned in school: BODMAS, or brackets, open; divide; multiply; add; and subtract, in that order.

It’s also known as PEMDAS, or parentheses first, exponents (ie powers and square roots) next, multiplication and division (left-to-right), and addition and subtraction (left-to-right).

## More Math Problems

Another viral math problem is:

There were some people on a train.
19 people get off the train at the first stop.
17 people get on the train.
Now there are 63 people on the train.
How many people were there on the train to begin with?

Here’s the answer to the problem:

There are 63 people on the train, meaning that one would subtract the 17 people who boarded the train.

Then you would have to account for the 19 people who got off, and one then has to add 19 and 46.

That gives 65, the answer, meaning there were 65 people on the train to start.

Or one could subtract 19 people by 17 people, which equals 2. Then add the other 63 to 2, which equals 65.

Louise Bloxham posted the problem to Twitter, confusing a number of people by stating the answer is 46. But that only solves the first portion of the problem.

As the Metro noted, “people said that the 19 was a red herring, which doesn’t make sense, or that the train was empty to begin with, which doesn’t make sense. It even says there are ‘some people’ on the train.”