[iamjochem]

given that "Ruby Programmer" >= "PHP Programmer" (not necessarily true, but be honest PHP is considered pretty low on the food chain around here :)), and given that I have no CS or Math background, I whipped up an answer to this in about 5 mins:

    https://gist.github.com/4611869

Even taking into account the effect of 'nerves' in an interview setting, on the face of things the 10% mentioned seems a bit depressing.

I do wonder how relevant or helpful this particular exercise/test is with regard to accurately gauging a candidate's potential efficacy within [a|your] development team. My instinct suggests to me that being able to write an algorithm under scrutiny and/or pressure is actually a fairly low priority skill (in most development shops) when compared to the other many other skills that are required to be a successful senior developer.