If you thought the Singaporean logic puzzle was tough, brace yourself for this math problem that was originally set for eight-year-old students in the Vietnamese town of Bao Loc. It’s apparently even stumped someone with a doctorate in economics with mathematics.

As Alex Bellos points out on The Guardian , it’s not a theoretically difficult problem. You just need to add in each of the digits from 1 to 9 so that the grid makes sense. (Update: those who are asking, the ‘:’ symbols signify division.) But in practice it turns out to be prrrrrretty tough. Good luck—and how about posting your solutions below? [VN Express via Guardian]

Image from VN Express