- the name of the puzzle may be significant
- the answer is always an even number (or zero)
See if you can work out the solution. It took me quite a while: twenty minutes perhaps. Some get it in seconds. There is some debate about whether the ease with which you solve the puzzle reveals something about the type of intellect you have. Some very clever people take a long time. Some people think the cleverer you are the longer it will take.
There is a metafilter discussion of the puzzle here, with over a hundred comments. Some of the comments give bigger or smaller clues as to the solution. Some give it away.
There is a discussion of how Bill Gates solved the 'petals round the rose' puzzle here. It doesn't tell you the solution.