Typically we'll start with an even simpler problem before dropping them into something like this. Over time, we've found that a good 1/3rd of candidates can be eliminated with just fizzbuzz (print the numbers 1-100, for numbers divisible by 3, print fizz. For numbers divisible by 5, print buzz. For both, print fizzbuzz). Even if they do stellar on a phone interview, a large number of candidates still cant get past this simple problem. It's also easy enough that you can give it to people at job fairs on the spot. Any programmer that you'd want to hire can finish this in less than 5 minutes, which is an easy time investment to make.