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 *

## DISCUSSION

There are other possible solutions, but the first one I came up with was: 6, 9, 3, 5, 2, 1, 7, 8, 4 in that order. So you do the multiplication and get 6+117:3+5+24-1-11+56:4-10=66. Then the division and get 6+39+5+24-1-11+14-10=66. Then the addition and get 88-22=66. 66=66. How I got to it was assigning each blank to a letter and writing the whole thing as an equation then grouping like terms. So... a+13(b/c)+d+12e-f+(gh/i)-21=66. Then you add 21 to both sides and a+d-f+13(b/c)+12e+(gh/i)=87. And since you’re limited to plugging in numbers between 1-9 for a-i, you see that there’s only so many combinations that will yield numbers close to 87. So then you start guessing and checking how large you have to make the various numbers to make it work. You need smaller numbers in the denominators and the subtraction and larger numbers being multiplied. I’m not going to find all possible solutions or know of a mathematical way to do this. I spent more time writing this explanation than solving the problem. Maybe 15 minutes total.