Well, for context... This is for hiring a freelancer on oDesk, and he's paying them to do the homework. It seems like a pretty fair way to evaluate a candidate.

If you have a look at his examples, they seem like reasonable questions too; they're not "turning a string of numbers into math problems", they're practical demonstrations of applications of the different technology stacks. If I were hiring a freelancer for a project, I'd probably do something like this with the option of just showing me some past work instead.

I've honestly never hired a carpenter or mechanic for a business before; from what I understand though there's usually a short probation period at the start where they're paid but can be let go summarily. This is the same kind of situation, only it's a shorter period.