Of course, the issue is not that folks on the internet have a hard time solving this particular math puzzle. As the story at Mental Floss indicates, there's more than one way to do it and neither of them are just plain and simple.
No, the issue is that this poser was an extra credit problem on a first grade math test in Singapore, if the original poster is to be believed. It's possible, although I'm a little skeptical. But I might not have come up with anything like a decent guess in grade one through 12, so probably don't go by me.