Hacker News Reader

Comment by LanceH | Hacker News Reader

I answered one of those mind benders nearly immediately in an interview. I figured out the trick as they were telling it basically. They asked me if I had heard it before. I hadn't. A final interview with the CEO and he asked if I had heard it before and I said I hadn't. I later realized based on how they were asking if I had heard it before, they thought I was lying.

The question: You get on a ski lift going up. It's 1000m long and you move at 20m/s. Between when you get on at the bottom and you get off at the top, how many of the chairs going down do you pass?

View on HN

I hate this question because it has an implicit bias that most likely the interviewer never saw. It's biased towards people who have ridden on a ski lift, which generally means someone wealthier.

If you're a poor kid who's never been skiing, you'll have a harder time conceptualizing the answer.

Is that sarcasm?

No.

Too bad. People encounter all forms of real discrimination but ski-lift discrimination sound really contrived.

To be fair to them, they did ask if I was familiar with a ski-lift and how it worked. I believe they even drew a basic picture.

So... the answer is "impossible to know"? There's no data on how many seats there are, their distance, or whatever. There could theoretically be literally just the seat you're sitting on in the entire system. Then the answer is zero.

Let's neglect the horizontal distance the chair(s) travel at the bottom and at the top of the lift. Then the answer is "all of them," which holds true in your one-chair scenario.

The question reminds me of "how many groves are there on a vinyl record?"

That depends: https://en.wikipedia.org/wiki/The_Monty_Python_Matching_Tie_...

I never knew! Thanks for sharing!

And how many is "all of them"? Btw it's not all, you're sitting on one, clearly you're not gonna meet that one.

If I ask you how many sugars you want in your coffee and you answer with an equation I will assume you're joking or an idiot :)

Since I don't drink coffee, you are unlikely to ask me that question. But if you did, I might assume you are either joking or an idiot ;).

Clearly, it's a trick question (with unnecessary details to distract you from a possible solution) and the one asking probably thinks they are very clever. So now, whether you accept an equation or insist that it's impossible to solve when it's not really comes down to how you want to play that game.

This seems like a stupid answer for it to be n-1, but it also seems like a stupid question. What's the answer?

It's n-1. I should have put the number of chairs in the question. I wasn't really trying to quiz everyone here, just identify which mind bender it was.

The "twist" on the answer is that you start below all the other chairs and finish above all the other chairs, so you must have passed them all along the way. I saw this immediately and answered "99" or whatever it was. In retrospect, I think they couldn't conceive of anyone figuring it out on the fly.

Is the answer zero chairs, since as far as we know there is only one chair on this lift?

could also be zero if it doesn't have any chairs at all because it's either a t-bar or standing gondola type.

I suppose you assume the chairs are at even distances?

Imagine there's a circle with marks evenly spaced, eg roots of unity. When you rotate it halfway, the point that started at the bottom has passed (vertically) all the other points ahead of it, because at some stage each point has reached the top and started going down.

I assume telling you a speed and not telling you how many chairs there are is intentional?

Assuming an "ideal ski lift" isn't it just all of them?

Is the answer that you pass all but one of the chairs going down?

You pass all of the chairs that are going down?

I assume the answer is all of them?

Do you pass them all?

It could be sum of all the chairs -1, could be -2 if the chairs are two per row, could be that the hub where they turn around has a large radius and the spacing of the chairs is close enough of that you don't "pass" them, could be 0 if there aren't any other chairs.

I suspect the answer to this is "not enough information", which, I suppose, hints at an applicants requirements gathering thought process?

Reply on news.ycombinator.com