I’m not sure where it came from originally. This one was posted by the prolific Cliff Pickover without credit.
The interesting thing about this problem is that, as the description says, it can be solved with only a pre-schooler’s knowledge. The apparent complexity hides a deeper simplicity that tricks a more educated person into trying all kinds of arthimetic and cyphers to solve it.
After solving it myself in about 5–10 minutes, showing just how childlike I am, I gave this problem to each of my three children: aged 19, 16, and 8.
The 19 year old gave up after seven minutes. The 16 year old persisted for about half an hour before giving up. The 8 year old took a look and, scratching her head, didn’t venture to try to solve it.