This is why you never end up hiring good developers - Quartz
Interesting article about job interviews. It says so many true things... Sometimes it's easy to answer some question in an interview, other times it's really hard to come up with a great solution to a stupid mathematical problem that doesn't prove anything. But the truth is that this doesn't say much about the people that you are hiring.
Much more important than what they know is how they learn it, and how quickly.
The difficulties of learning things and understanding unknown things are traits that define people. You should look for that.