Interview puzzle questions

So, for some random reason the other night we (I will say no more than we were a small group and out drinking) started to talk about using puzzle questions in interviews.

One question that a friend uses goes something like this:

• In a tournament where there are 80 teams how many matches (where a match is between two teams) are played before the winner is decided ?

Personally, I don't like using mathematical puzzles in interviews as I feel they are quite artificial (I remember having the hardest time with some pure maths in uni but no problem at all when seeing the same ideas in quantum theory (ok, so you could argue that I had already been exposed to them, but I do think that framing alone (e.g. abstract vs practical) can quite significantly alter the ability of someone to "get a grip" on the problem at hand - and let's face it there are lots of people who don't think the same way (I know this from personal experience :-) ).

Anyway, my gut feeling is that if you get the answer right then the interviewer assumes you had seen it before (as my friend did when I worked it out mid-pint) and if you get it wrong they use it to flatter their own ego.

Recently, I have been using a method inspired by "Blink", the book written by Malcolm Gladwell of "Tipping Point" fame, which involves chatting to the person in a general way and then after 15 minutes going with your gut feeling. This actually worked surprisingly well (and I felt really good as I had gained 45 minutes from the process!). I know it worked well by comparing notes with a couple of other people who were using more orthodox techniques....

I will write up my thoughts on the Gladwell books another time, but for the moment I would like to confess that I actually like puzzles. However, I'd like to qualify that by saying that I don't think they are appropriate for interviews unless they are frighteningly hard and you get the interviewee to verbalise their approach to the problem.

Here are some mathematical puzzles in "gradually" increasing level of difficulty:

• there are three submarines and a ship. Each submarine fires a torpedo at the ship, and each has a 30% chance of hitting it. What is the probability that the ship is hit ?
  
• you are given many matches and two pieces of string that take one hour to burn but do not burn at a uniform rate - how do you time 45 minutes ?
• differentiate x to the power x with respect to x
  
• make a wireframe cube from eight one ohm resistors - what is the resistance between diagonally opposite corners ?
• repeat 10 times: toss a coin - if heads put a white marble in an urn otherwise a black marble. Someone who knows the population process then selects a marble at random from the urn, examines it, and then returns it. They do this 10 times. If all the drawn marbles were white what is the probability that the urn only contained white marbles ?
  

Actually I have an additional confession: an interviewer once asked me the burning string question. Not liking these questions in interviews I replied that I would attempt it if they attempted the wireframe cube one for me (thinking it only fair that I get to evaluate their abstract problem solving ability if they are looking to do the same to me)......the interview didn't last very long :-)

After getting through this long post you need some light entertainment. What about the screen tests for the roles in star wars (you will need to hit play twice, about 5 seconds apart - but this was better than the autostart it would do otherwise) ?