Viral Math, Part Deux

Singapore’s mind-bending logical riddles are so last month. Enter: Vietnam, the latest country to be swept up in what could easily be known as “the viral-math epidemic of 2015.”

This one might even trump its Singaporean predecessor, which became a global legend earlier this year. That quandary, for those who aren’t familiar with it, asked fifth-graders to figure out the birthday of a certain “Cheryl,” who gave two of her friends—“Albert” and “Bernard”—a list of 10 possible dates. She then privately told Albert the month, and Bernard the day. (“Albert: I don’t know when Cheryl’s birthday is, but I know that Bernard does not know too. Bernard: At first I don’t know when Cheryl’s birthday is, but I now know. Albert: Then I also know when Cheryl’s birthday is.”)

The question: So when is Cheryl’s birthday, really?  The befuddlement that ensued, first among angry parents and then among Internet users around the world, prompted a #cherylsbirthday Twitter trend. Many wondered whether it even had to do with math.

The Vietnam question, however, is based on simple arithmetic; it has a Sudoku-esque feel. Unlike the Singaporean conundrum (which an investigation later found was actually meant for ninth-graders and had been designed to “sift out the better students”), this one isn’t aimed at deliberately confusing kids. It’s just really tough. According to reports, the question in question is designed for third-graders (i.e., 8-year-olds) in the town of Bac Loc in the Vietnamese highlands—students who, as one teacher told a local news outlet, tend to have lower proficiency levels than their peers in more developed parts of the country. (You’ll find the actual question, and a link to its solution, further down.)

The Internet popularity of these foreign math questions is a curious phenomenon—particularly in the U.S., where according to one study most middle-schoolers prefer eating broccoli to doing math. Roughly a third of Americans, meanwhile, say they would rather clean the bathroom than solve an arithmetic problem. Yet when it comes to a strange and unusual math question for children from afar, it seems that Americans, and people around the world for that matter, can’t resist the temptation to take a stab. It’s no wonder Are You Smarter Than a Fifth Grader? is such a hit show.

It isn’t surprising that students in Vietnam are being asked to solve such tricky math questions. Vietnam is one of the highest-achieving countries in the world when it comes to math and science, placing 12th out of 76 countries in new rankings by the OECD, which conducts the global proficiency exam known as the Programme for International Student Assessment. The same survey showed that nearly a quarter of America’s 15-year-olds struggle to successfully complete basic math and science tasks. The U.S. was ranked No. 28 on the list.

Now, to the question: To solve it, the blanks in the “snake” must be filled with any number between one and nine so that the equation works. Each number can only be used once.

The Guardian’s resident math expert Alex Bollas’s posted a solution on Thursday. His hint? “We tame the Vietnamese snake by a process of trial and error, making educated guesses as we go.” Oh, and it helps to rewrite that “snake” out as an equation.