No, really: stop me if you’ve heard this one before

Silicon Alley Insider is running an article titled 15 Google Interview Questions That Will Make You Feel Stupid. Several of the questions are interesting, but several are chestnuts that if you know puzzles, you barely remember where you heard them the first time. On the other hand, if you aren’t a puzzle person you’re not likely to solve them quickly and under pressure during a job interview, making them less than useless. It doesn’t help that the article doesn’t display a good understanding of the puzzles, and gets several answers wrong.

Here they are:

Stupid Interview Questions

Wanted: Boys

In a country where people want only boys every family continues to have children until they have a boy. If they have a girl, they have another child. If they have a boy, they stop. What is the proportion of boys to girls in the country?

This one is perhaps interesting from a can-you-be-misled standpoint: since the probability of any given child being a boy or girl hasn’t changed, the overall proportion can’t be changed by decisions you make after the child is born. It’s a nice puzzle, but I first heard it at least ten years ago. If you know puzzles, you (almost certainly) know this one.

Manhole Covers

Why are manhole covers round?

Seriously? This is supposed to make you feel stupid? Again, if you have any interest in puzzles whatsoever, you’ve heard this one before. The answer to give is definitely not the one they give — so they can’t fall in — at this point that’s trite. The trick now is to come up with some other answer that has at least some plausibility: making them round makes it easier to move them by rolling them since they’re heavy; a round hole makes sense from a “get the most access for the least hole” standpoint; round holes are sturdier and easier to dig; a circular cover doesn’t need to be oriented before being put in place. Or point out that any shape with a constant diameter will prevent falling in, the most common being the reuleaux triangle. Or if you want to be cheeky point out that not all countries use round manhole covers.

Handy

How many times a day does a clock’s hands overlap?

I actually like this one, old as it is. For some reason the answer never sticks in my mind (now that I’ve said that it will) and I have to think it through. Sometimes I even get it wrong

Thinking Laterally

A man pushed his car to a hotel and lost his fortune. What happened?

A classic lateral thinking (or situation) puzzle. Generally these are followed by a series of yes/no questions to attempt to figure out the intended meaning of the misleading original clue. Obviously the interviewer’s goal with this is more to see how clever you are at asking questions rather than how quickly you can solve the puzzle; but again, if you know lateral thinking puzzles this one isn’t going to pose much of a challenge (I solved it after about fifteen seconds, obviously without asking any questions, and I’m not even very good at lateral thinking puzzles), and if you don’t know about them, you’re unlikely to make much of an impression with your question-asking.

Logical Pirates

You’re the captain of a pirate ship and your crew gets to vote on how the gold is divided up. If fewer than half of the pirates agree with you, you die. How do you recommend apportioning the gold in such a way that you get a good share of the booty, but still survive?

The answer they give here is just plain wrong. The wording of the puzzle itself is weak as well. This puzzle is at least five years old (albeit a good puzzle) and fairly well-known. It’s ridiculously hard to solve on the fly and fairly likely to be known by anyone interested in puzzles. The puzzle is usually put something like this:

There are five pirates on a ship, with 100 gold pieces to divide between themselves. The pirates are logical, self-preserving, greedy, and bloodthirsty in that priority. They agree that they will each in turn propose a way to divide up the gold, starting with the captain. If the majority of the pirates agree with the proposal, that is how the gold will be divided. Otherwise the pirate making the proposal will be tossed over, and the next in terms of seniority will make a new proposal. What proposal should the captain make?

To find the answer, number the pirates and work backwards:

4. If it comes down to pirates 4 and 5, then 4 is out of luck; even if he proposes giving all the gold to 5, 5’s bloodthirsty and he’ll say no, kill 4, and take all the gold.

3. If it comes down to 3, 4, and 5, 4 will vote yes to 3’s proposal no matter what, in order to save his own life (see 4 above). Therefore 3 can propose that he (3) gets 100 pieces of gold and 4 will vote yes, and the proposal passes.

2. If it comes down to 2, 3, 4, and 5, since 4 and 5 get nothing under 3’s proposal, 2 can propose 98 pieces of gold for himself and 1 each for 4 and 5, and they will vote yes, giving a three-to-one majority.

1. Therefore the captain, pirate 1, should propose 97 pieces of gold for himself, 1 piece for 3, and 2 pieces for either 4 or 5, and have a three-to-two majority.

Ballsy

You have eight balls all of the same size. 7 of them weigh the same, and one of them weighs slightly more. How can you find the ball that is heavier by using a balance and only two weighings?

Seriously, I first encountered this puzzle in a book my parents gave me when I was eight. Guess I should have applied to Google back then, eh?

Egg Drop Soup

You are given 2 eggs. You have access to a 100-story building. Eggs can be very hard or very fragile means it may break if dropped from the first floor or may not even break if dropped from 100th floor. Both eggs are identical. You need to figure out the highest floor of a 100-story building an egg can be dropped without breaking. The question is how many drops you need to make. You are allowed to break 2 eggs in the process.

As with the pirate puzzle, the answer given here is incorrect. The correct answer is 14 drops.

You start on the 14th floor; if the first egg breaks there you start at the first floor with the second egg and work your way up each floor, potentially to the 13th floor.

If the first egg survives the 14th floor, you go to the 27th: thirteen (not fourteen) floors higher because you burned a drop on the 14th floor. If the first egg breaks there you start at 15 and try each floor from there potentially up to 26.

This pattern continues. If the first egg survives, you go up twelve floors to the 39th, eleven to the 50th, and on to the 60th, 69th, 77th, 84th, 90th, 95th, 99th, and finally the 100th.

Note that 13 drops is not possible; if you start on the 13th floor (and the first egg survives) you will reach the 88th floor, then the 90th, then the 91st, and be out of luck.

Shrinky-dink

You are shrunk to the height of a nickel and your mass is proportionally reduced so as to maintain your original density. You are then thrown into an empty glass blender. The blades will start moving in 60 seconds. What do you do?

The answer given here is wrong. “This one is all about the judging interviewee’s creativity.” No. This is all about understanding physics. Again, this is a common puzzle. As any seasoned puzzler will tell you, if you maintain density and proportions, your strength decreases as the square of the change, but your mass decreases as the cube of the change. So you would simply jump out of the blender. Trying to break the electric motor as suggested would be far less possible than it would be if you were full-sized, i.e. impossible (since you wouldn’t be able to throw it).

Conclusion

Interview puzzles fail on two fronts: for anyone who has a taste for puzzles, you’re more likely to give them a puzzle they already know than to truly stump them; and for those who don’t happen to like puzzles, your attempt to find out how clever they are is more likely to simply frustrate them and mislead you. Finally, asking more difficult/obscure puzzles perhaps solves the first issue, at the cost of making the second issue far worse.

That said, I love interview puzzles in general, so bring ‘em on!