Press J to jump to the feed. Press question mark to learn the rest of the keyboard shortcuts
7
Posted by8 years ago
Archived

## What are some of your favorite logic or tricky probability questions?

I'm helping to create a resource for people to come and take logic and probability questions to test themselves or to practice for interview situations. I would love to hear from the reddit community with your favorites (with answers hopefully).

21 comments
75% Upvoted
This thread is archived
New comments cannot be posted and votes cannot be cast
Sort by
level 1
4 points Â· 8 years ago

You should totally include the Monty Hall problem.

level 2
2 points Â· 8 years ago

I fucking hate it when people won't believe me on this one.

level 3
[deleted]
-1 points Â· 8 years ago

I don't like this one. Because, sure, in a purely mathematical sense the probabilities HAVE changed because you're able to pick two out of 3 doors instead of 1/3 doors. But it never changes what door the goat/car/midget is behind. You've changed nothing about the scenario. Regardless of the odds.

level 4
3 points Â· 8 years ago

Scale it up to 1,000,000 doors.

You pick one door.

The host opens 999,998 others.

The obvious thing to do is to switch.

level 5
[deleted]
0 points Â· 8 years ago

I disagree. While the visual for odds-adjustment is more obvious, it still has absolutely no effect on which door the goat is behind.

level 6
1 point Â· 8 years ago

In your first choice, you had a one in a million chance of choosing the right one.

When he opened all but 2, the door you didn't choose now has a 50:50 chance of being the one with the car.

level 7
[deleted]
0 points Â· 8 years ago

In your first choice you had a 50:50 chance on choosing the right one based purely on outcome. The odds were one in a million. But there were always ever only two outcomes. Goat, or no goat.

After he's opened up all but two doors the odds have changed now. As soon as it's offered you're technically picking doors again. Picking the same door is a decision with the same odds as switching doors. 50:50. Because switch or no switch you have two doors 'now.' And you're picking one of them 'now.'

level 8
1 point Â· 8 years ago

In your first choice you had a 50:50 chance on choosing the right one based purely on outcome.

By your logic, if I buy a lottery ticket I have a 50:50 chance of winning, purely based on outcome. I wish that that were the case.

http://www.theproblemsite.com/games/monty_hall_game.asp

Leave it on the whole day. See what you get.

level 4
[deleted]
2 points Â· 8 years ago

Indeed, you've just gained information which allows you to make a more informed guess.

level 4
Comment deleted8 years ago(2 children)
level 5
1 point Â· 8 years ago

The problem is that the question is often described ambiguously.

level 5
1 point Â· 8 years ago

Unfortunately, most people are unable to do this.

level 1
4 points Â· 8 years ago

Ok, everyone has heard it, and I don't even know if it's logic, but I will try and give you a hand. Please just stop reading and don't bitch in the comments when you have heard it, god damn.

So, three guys are on a road trip and it's getting late so they find a shady hotel on the side of the road, good enough to spend the night. They walk in, the guy behind the counter greets them and exclaims "Rooms are \$30 each!". So the three guys say perfect, and cough up \$10 each and head to their room for some rest. Well it's at this point that the hotel clerk thinks "Fuckballs, today is Thursday, which means rooms are five dollars off" So he knocks on this small wooden door, where his midget slave lives. He gets the midget to run upstairs and return the five sheet to the guests.

So the Midget, Milda, gets to the room and explains the refund and returns the five dollars. Well the three guys look at each other and say hmm, well we can't divide the five dollar between three peopls, so we will take 1 dollar each, and give two dollars back to Milda. Milda is very happy and walks away, but then they start to think:

Each person paid ten dollars, but got one dollar back. So that makes a payment of \$9 each, or \$27, for the hotel room, plus the \$2 they gave to the midget, to total \$29. Where did the other dollar go?

level 2
2 points Â· 8 years ago

The trick is how the question is worded. Of the original \$30, they paid \$25 for the hotel room, gave \$2 to the midget, and received \$3 back. The problem counts the midget's \$2 twice.

level 2
1 point Â· 8 years ago

What the fuck, man. That's not a logic problem. It's three people making a totally obvious mistake.

level 1
[deleted]
2 points Â· 8 years ago

You know a man has two children, and you're curious whether both of his children are boys.

a) Suppose he tells you that his older child is a boy. The probability that both children are boys is going to be 1/2, since the possibilities will be either BG or BB. Easy enough.

b) Suppose instead he tells you that he has at least one son. The possibilities now will be either BB, BG, or GB (GG is eliminated), so now it's only a 1/3 chance that both his children are boys. Surprising, perhaps. But nothing compared to:

c) Suppose instead that he tells you that he has at least one son born on a Tuesday. Completely unrelated, right? But now the odds that both are boys are actually 13/27. Graph it out for yourself with all the days of the week: there will be 13 BB possibilities, 7 BG possibilities, and 7 GB possibilities. This is because there is a certain redundancy for BB at Tuesday; it really covers two different "possibilities" depending on ordering. It's tough to word clearly, and the kind of thing you won't believe unless you graph it yourself anyway. =\

level 1
1 point Â· 8 years ago

project euler is awesome

level 2
1 point Â· 8 years ago

Most of them require some deeper mathematical skills and are too tough for the general population to solve with pencil and paper.

level 3
Original Poster1 point Â· 8 years ago

I agree. I like project euler but I'm trying to put together questions that can be solved quicker and by people with less programming and/or math skills.

level 1
1 point Â· 8 years ago

Anything that starts out with the phrase, "Negate the following statement:..."

level 1
[deleted]
1 point Â· 8 years ago

Which is worth more, getting a yearly raise of \$2000 every year, or a 6-month period raise of \$500 (In other words, if you make \$10000 the 1st 6 months, you'd make \$10500 the next 6) every 6 months.

Let's say you make \$20000 a year to start, or \$10000 every 6 months.

......Raise every 6 mos, raise every year
1st 6 mos.... \$10000 .... \$10000
2nd ........... \$10500 .... \$10000
3rd ........... \$11000 .... \$11000
4th ........... \$11500 .... \$11000

As you can see, you'll make \$500 more every year by taking the "smaller" raise.

Edit: I don't know how to make an actual table.

level 2
1 point Â· 8 years ago

woah! Slope!

level 2
1 point Â· 8 years ago

I think you made a typo - you must have meant \$1000 every year, not \$2000

level 3
[deleted]
1 point Â· 8 years ago

nope. If you make \$20,000 the first year, and \$22,000 the second year, then you make \$10,000 the 1st and 2nd 6 months, then \$11,000 the 3rd and 4th 6 months.

Community Details

19.9m

Subscribers

103k

Online

/r/AskReddit is the place to ask and answer thought-provoking questions.

r/AskReddit Rules
1.
Rule 1 - Questions must be clear and direct
2.
Rule 2 - No personalized advice requests
3.
Rule 3 - Open ended questions only
4.
Rule 4 - No personal info
5.
Rule 5 - No loaded questions
6.
Rule 6 - No questions seeking professional advice
7.
Rule 7 - No begging for goods or services
8.
Rule 8 - No demeaning terms or personal attacks
9.
Rule 9 - No image only replies or large ascii art
10.
Comments in [Serious] posts must be on topic
Filter by Post Type
Other Resources

Other useful resources

Â© 2018 Reddit, Inc. All rights reserved
{"found_math": false, "script_math_tex": 0, "script_math_asciimath": 0, "math_annotations": 0, "math_alttext": 0, "mathml": 0, "mathjax_tag": 0, "mathjax_inline_tex": 0, "mathjax_display_tex": 0, "mathjax_asciimath": 0, "img_math": 0, "codecogs_latex": 0, "wp_latex": 0, "mimetex.cgi": 0, "/images/math/codecogs": 0, "mathtex.cgi": 0, "katex": 0, "math-container": 0, "wp-katex-eq": 0, "align": 0, "equation": 0, "x-ck12": 0, "texerror": 0}