At my last job, we asked two simple questions and many "senior developers" couldn't get them:

1. Write a function that determines if a number is prime. If they didn't know what a prime number was, we would tell them.

2. A simple problem that required designing a database schema and sql query that involved a left outer join.

There were a lot of developers who couldn't do it.

On the other hand, there was one developer who was just learning c#, who had spent most of his time doing VB.net, couldn't answer a lot of the technical questions but we could tell by his thought process and how he explained real world problems he solved that he would be a great asset to the company. We fought for him over more "senior" developers.

When I have a chance to hire again, I'm going to fight to get him -- even though he sucks at interviewing and i might have to do a little convincing.

> Write a function that determines if a number is prime. If they didn't know what a prime number was, we would tell them.

Did you also give them an algorithm to implement or was a brute force method good enough?

First the brute force algorithm. Then we would ask them how could they optimize it. A lot of people couldn't even get to the brute force algorithm.

1st level optimization: skip even numbers.

2nd level optimization that I was the only person to get when I had to interview for the company: loop starting at 2, and go the square root of the argument.