If you thought the Singaporean logic puzzle was tough, brace yourself for this maths 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 numbers, from 1 to 9, so that the grid makes sense. 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