Interview Questions: Practice Brainteasers

Practice Question #1

Why are manhole covers round?

The classic brainteaser, straight to you via Microsoft (the originator). Even though this question has been around for years, interviewees still encounter it. Here's how to "solve" this brainteaser. Remember to speak and reason out loud while solving this brainteaser!

Why are manhole covers round? Could there be a structural reason? Why aren't manhole covers square? It would make it harder to fit with a cover. You'd have to rotate it exactly the right way. So many manhole covers are round because they don't need to be rotated. There are no corners to deal with. Also, a round manhole cover won't fall into a hole because it was rotated the wrong way, so it's safer.

Looking at this, it seems corners are a problem. You can't cut yourself on a round manhole cover. And because it's round, it can be more easily transported. One person can roll it.

Practice Question #2

The power has gone out in your hotel room and it is pitch black. You have 11 white socks and 10 black socks in your suitcase. (It's a long-term engagement.) You must put on a matched pair of socks, or you'll look terrible at your presentation! How many socks must you pull from the drawer to be assured of a matched pair?

Don't be fooled! Either white or black will do, so you need only three socks to be sure of either a white or black matched pair.

Practice Question #3

You are in a room with three light switches. Each controls one of three light bulbs in the next room. You must determine which switch controls which bulb. All lights are off. You may flick only two switches and enter the room with the light bulbs only once. How would you determine which switch controls which light bulb?

This is an invitation from the consulting firm to express your "out-of-the-box" thought patterns. Staring in disbelief, whimpering in fear, or otherwise reacting negatively will shoot down your chances. So be creative. Ask if you can pull out your cell phone and call a pal for assistance. Run out and buy a drill so you can peek through the wall. There is, however, one especially elegant solution. Turn one light bulb on for about twenty minutes. Then turn it off. Turn another switch on. Then enter the room and feel the two bulbs that are off. The hot one will be attached to the switch that you just turned off. Using all your senses - that's thinking like a consultant!