# A math riddle for you. Feel free to submit your own. **URL:** \ **Category:** Miscellaneous and Personal Stuff I Must Share\ **Created:** [October 19, 2018, 12:15pm UTC](https://boards.straightdope.com/t/a-math-riddle-for-you-feel-free-to-submit-your-own/823205 "2018-10-19T12:15:49Z")\ **Posts on this page:** 17\ **Page:** 6
**Author:** ![begbert2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/begbert2/32/3357_2.png) [@begbert2](https://boards.straightdope.com/u/begbert2)\ **Post date:** [October 24, 2018, 8:38pm UTC](https://boards.straightdope.com/t/a-math-riddle-for-you-feel-free-to-submit-your-own/823205/101 "2018-10-24T20:38:10Z")
> [@CookingWithGas](#): > > You can’t switch to a door once he opens it. :dubious: Since when? Not in any statement of the puzzle _I’ve_ seen - though in all those it’s made clear that he’s not going to ever open the car’s door (or the door you picked, for that matter). If Monty could pick the door with the car and then start pointing and laughing at you for not picking the winner, then yeah, switching to the other loser door won’t help you. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [October 24, 2018, 8:55pm UTC](https://boards.straightdope.com/t/a-math-riddle-for-you-feel-free-to-submit-your-own/823205/102 "2018-10-24T20:55:15Z")
> [@begbert2](#): > > > [@CookingWithGas](#): > > > > You can’t switch to a door once he opens it. > > :dubious: Since when? Not in any statement of the puzzle _I’ve_ seen Since always. Have you ever even seen the show? The puzzle always specifies that the player may stick with the original choice or switch to the other unopened door. [Wikipedia version](https://en.wikipedia.org/wiki/Monty_Hall_problem) [Marilyn’s site](http://marilynvossavant.com/game-show-problem/) [Better Explained](https://betterexplained.com/articles/understanding-the-monty-hall-problem/) [Wolfram](http://mathworld.wolfram.com/MontyHallProblem.html) [Scientific American](https://www.scientificamerican.com/article/the-3-door-monty-hall-problem/) [Rosenhouse, Jason, _The Monty Hall Problem_, Oxford Press, 2009, p. ix](https://www.amazon.com/Monty-Hall-Problem-Remarkable-Contentious/dp/0195367898) ---
**Author:** ![begbert2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/begbert2/32/3357_2.png) [@begbert2](https://boards.straightdope.com/u/begbert2)\ **Post date:** [October 24, 2018, 9:02pm UTC](https://boards.straightdope.com/t/a-math-riddle-for-you-feel-free-to-submit-your-own/823205/103 "2018-10-24T21:02:57Z")
> [@CookingWithGas](#): > > Since always. Have you ever even seen the show? The puzzle always specifies that the player may stick with the original choice or switch to the other unopened door. > > [Wikipedia version](https://en.wikipedia.org/wiki/Monty_Hall_problem) > [Marilyn’s site](http://marilynvossavant.com/game-show-problem/) > [Better Explained](https://betterexplained.com/articles/understanding-the-monty-hall-problem/) > [Wolfram](http://mathworld.wolfram.com/MontyHallProblem.html) > [Scientific American](https://www.scientificamerican.com/article/the-3-door-monty-hall-problem/) > [Rosenhouse, Jason, _The Monty Hall Problem_, Oxford Press, 2009, p. ix](https://www.amazon.com/Monty-Hall-Problem-Remarkable-Contentious/dp/0195367898) And they also specify he ain’t gonna open the door with the car. If you’re going to change the problem so far to allow that as a possibility, then there’s no reason to assume that the phrasing that assumes you would never _want_ to pick the opened door was actually a rule preventing it. Suffice to say if Monty can go around opening the car door whenever possible so he could mock you for not literally tackling him and stopping him from doing so, it changes the problem significantly. If he demonstrates a marked tendency for doing this, then you should clearly not switch, because the fact he didn’t mock you with the car suggests he couldn’t because you already had picked it. (Unless he’s allowed to open your door too, in which case you’re boned in all cases.) ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [October 24, 2018, 10:29pm UTC](https://boards.straightdope.com/t/a-math-riddle-for-you-feel-free-to-submit-your-own/823205/104 "2018-10-24T22:29:44Z")
> [@Mr\_Shine](#): > > A man offers you a car if you can choose which one of three doors it’s behind. Behind the other 2 are goats. After you choose, he opens one of the other doors and reveals a goat. He then offers you the chance to switch doors. Should you, or does it not matter? > > You should definitely not switch. The only reason someone would add this option in after the fact is if you chose the car straight away. > [@That\_Don\_Guy](#): > > Ah, the old Monty Hal-er, Wayne Brady problem. > > And the answer depends on semantics - specifically, when it says “he opens one of the other doors and reveals a goat,” is it clear that he was going to make sure that he opened a door with a goat, or did he open a door at random and it just happened to have a goat behind it? Actually, if you read **Mr Shine** ’s spoilered answer, it’s pretty clear that he did not intend his version to be the same as the classic Monty Hall problem, though you’re maybe supposed to think it is. In **Mr Shine** ’s version, if your original choice had had a goat, “the man” would not have offered to let you switch. For the record, I don’t like **Mr Shine** ’s version of the question. If “the man” is that self-serving, why is he offering you the chance to win a car in the first place? ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [October 24, 2018, 11:06pm UTC](https://boards.straightdope.com/t/a-math-riddle-for-you-feel-free-to-submit-your-own/823205/105 "2018-10-24T23:06:21Z")
> [@begbert2](#): > > […]there’s no reason to assume that the phrasing that assumes you would never _want_ to pick the opened door was actually a rule preventing it. The phrasing says you have the option to switch to the other unopened door. That’s actually a rule. In six cites. ---
**Author:** ![begbert2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/begbert2/32/3357_2.png) [@begbert2](https://boards.straightdope.com/u/begbert2)\ **Post date:** [October 24, 2018, 11:08pm UTC](https://boards.straightdope.com/t/a-math-riddle-for-you-feel-free-to-submit-your-own/823205/106 "2018-10-24T23:08:58Z")
> [@CookingWithGas](#): > > The phrasing says you have the option to switch to the other unopened door. That’s actually a rule. In six cites. Tell you what - I’ll concede that the situation could, if you massively alter the problem in one specific way and then carefully avoid altering it in another way, become a significantly more stupid situation than it’s generally conceived. I think that’s a reasonable description of what we’re discussing here. ---
**Author:** ![Malleus\_Incus\_Stapes](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/malleus_incus_stapes/32/3293_2.png) [@Malleus\_Incus\_Stapes](https://boards.straightdope.com/u/Malleus_Incus_Stapes)\ **Post date:** [October 25, 2018, 9:07pm UTC](https://boards.straightdope.com/t/a-math-riddle-for-you-feel-free-to-submit-your-own/823205/107 "2018-10-25T21:07:23Z")
One that my father heard from one of his students: > [@](#): > > A mat of algae in a pond doubles in size every day. It takes forty days for it to cover the whole pond. How many days does it take for it to cover half the pond? Technically the answer is very simple. In retrospect, it’s even obvious. But every\* person I’ve told this to has given up, then done a massive :smack: when I told them the answer. _I_ gave up in a math-panic, then felt really stupid when my dad told me the answer. My father, who takes advanced mathematics courses for fun, had to have the student tell him the answer. \*As I was writing this, my brother just became the only person so far to get it, and on the first try, too. Whadaya know. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [October 25, 2018, 10:13pm UTC](https://boards.straightdope.com/t/a-math-riddle-for-you-feel-free-to-submit-your-own/823205/108 "2018-10-25T22:13:25Z")
> [@pulykamell](#): > > As a kid, one of my favorite misdirection groaner versions of this type of puzzle started with “Ok, you’re [or I’m] driving a bus. At the first stop X amount of people get on. At the next stop, half the people get off, and 3 people get on.” And then you just spin in with some types of seemingly complicated, but doable math for a couple rounds. At the end, the question is “what is the name of the bus driver?” > > Well, it’s there in the first sentence. > > My father drove me nuts, because I’ve heard my father tell that riddle, and completely forget the first sentence as, you know, kinda being the key to it all, so it just ended up completely non-sensical in the end. Or maybe he was trying to some abstract Kaufmann shit, I don’t know. 🙂 Always heard that one with the question being how old the bus driver is. Traps people who don’t think about what numbers actually mean in a problem. ---
**Author:** ![Reindeer\_Flotilla](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reindeer_flotilla/32/3024_2.png) [@Reindeer\_Flotilla](https://boards.straightdope.com/u/Reindeer_Flotilla)\ **Post date:** [October 25, 2018, 10:31pm UTC](https://boards.straightdope.com/t/a-math-riddle-for-you-feel-free-to-submit-your-own/823205/109 "2018-10-25T22:31:16Z")
> [@Malleus\_Incus\_Stapes](#): > > Technically the answer is very simple. In retrospect, it’s even obvious. The version I heard, which is more or less the same math riddle though whose answer is slightly less obvious, goes something like this: A certain thing can spontaneously reproduce once a day; starting with one it will take 30 days to reach 536,870,912 things. How many days would it take to reach that total if you started with two things? Like with your riddle most people mistakenly say 15 days or half the time, however the answer is: 29 days. In the case where you started with one thing, after one day you have two things and equaled the starting condition of the riddle. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [October 25, 2018, 11:09pm UTC](https://boards.straightdope.com/t/a-math-riddle-for-you-feel-free-to-submit-your-own/823205/110 "2018-10-25T23:09:48Z")
> [@CookingWithGas](#): > > Here is [one from Car Talk](https://www.cartalk.com/puzzler/porch-potatoes), abridged: > > RAY: Potatoes are 99 percent water. So say you take 100 pounds of potatoes and you set them out on your back porch to dry out. After a while, enough water has evaporated so that they are now 98 percent water. If you were to weigh those potatoes at that moment… > > TOM: 99 pounds. > > RAY: You are wrong. > > RAY: Now, unencumbered by the thought process as usual, my brother guessed 99 pounds. > > TOM: Yeah. > > RAY: Now, when I guessed, off the top of my head, I guessed about 90 pounds. > > TOM: 'Cause it just feels right. > > RAY: But if you do the math, 1 percent of 100 --which is what the potato is-- is one pound. As we told you, that’s 1 percent. So 2 percent, when it’s 98 percent water, two percent of the new weight of the mass is still going to be equal to that one pound, and 2 percent of 50 pounds is a pound. So the potato weight is now 50 pounds, not 100. Wait, what? ---
**Author:** ![begbert2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/begbert2/32/3357_2.png) [@begbert2](https://boards.straightdope.com/u/begbert2)\ **Post date:** [October 25, 2018, 11:15pm UTC](https://boards.straightdope.com/t/a-math-riddle-for-you-feel-free-to-submit-your-own/823205/111 "2018-10-25T23:15:07Z")
> [@Bear\_Nenno](#): > > Wait, what? In terms of percent-solids rather than percent-water, the potatoes were 1% solids before and are 2% solids after. 1% is 1/100, and 2% is 1/50. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [October 25, 2018, 11:32pm UTC](https://boards.straightdope.com/t/a-math-riddle-for-you-feel-free-to-submit-your-own/823205/112 "2018-10-25T23:32:45Z")
> [@begbert2](#): > > In terms of percent-solids rather than percent-water, the potatoes were 1% solids before and are 2% solids after. 1% is 1/100, and 2% is 1/50. Thanks, yea I just wasn’t seeing it. I had to get out paper and do it like this: total weight (y) = potato weight (p) + water weight (w) p = 1 w = .98y y=p+w y=1 + .98y y-.98y = 1 .02y = 1 .02y/.02 = 1/.02 y = 50 I simply wasn’t seeing it until I really dumbed it down… ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [October 26, 2018, 5:04pm UTC](https://boards.straightdope.com/t/a-math-riddle-for-you-feel-free-to-submit-your-own/823205/113 "2018-10-26T17:04:48Z")
> [@Bear\_Nenno](#): > > Thanks, yea I just wasn’t seeing it. I had to get out paper and do it like this: > total weight (y) = potato weight (p) + water weight (w) > p = 1 > w = .98y > > y=p+w > y=1 + .98y > y-.98y = 1 > .02y = 1 > .02y/.02 = 1/.02 > y = 50 > > I simply wasn’t seeing it until I really dumbed it down… Part of what I left out was RAY: Now you can solve this puzzler algebraically, and if you don’t solve it algebraically, you are going to get the wrong answer. ---
**Author:** ![begbert2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/begbert2/32/3357_2.png) [@begbert2](https://boards.straightdope.com/u/begbert2)\ **Post date:** [October 26, 2018, 5:11pm UTC](https://boards.straightdope.com/t/a-math-riddle-for-you-feel-free-to-submit-your-own/823205/114 "2018-10-26T17:11:14Z")
> [@CookingWithGas](#): > > Part of what I left out was > > RAY: Now you can solve this puzzler algebraically, and if you don’t solve it algebraically, you are going to get the wrong answer. I gotta say that even knowing how it works it’s still really unintuitive. But then fractions/percentages are always weird. B is 50% more than A. A is 2/3 of B. ---
**Author:** ![notfrommensa](https://avatars.discourse-cdn.com/v4/letter/n/f14d63/32.png) [@notfrommensa](https://boards.straightdope.com/u/notfrommensa)\ **Post date:** [October 27, 2018, 2:34am UTC](https://boards.straightdope.com/t/a-math-riddle-for-you-feel-free-to-submit-your-own/823205/115 "2018-10-27T02:34:03Z")
> [@x-ray\_vision](#): > > Not really. That’s what makes riddles such as that one fun for some people. They quickly figure (especially if asked orally) they just cut the age in half. After they’re told the answer, they slap their forehead because they fell for it. Here’s a non-math riddle told orally following that premise that is told by kids: > > Spell so (kid spells so) > Spell foe (kid spells foe) > Spell no (kid spells no) > > What do you do at a red light? > > Of course kids know that you stop. But if they fall for it… I think I would answer S-E-W for the first spelling quiz. ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [October 27, 2018, 5:09am UTC](https://boards.straightdope.com/t/a-math-riddle-for-you-feel-free-to-submit-your-own/823205/116 "2018-10-27T05:09:54Z")
> [@x-ray\_vision](#): > > Spell so (kid spells so) > Spell foe (kid spells foe) > Spell no (kid spells no) > > What do you do at a red light? > > Of course kids know that you stop. But if they fall for it… There was an episode of _CSI_ where Nick asked Gil why he didn’t get the promotion to CSI III. Gil Grissom: Repeat after me. Silk, silk, silk. Nick Stokes: Silk, silk, silk. Gil Grissom: What do cows drink? Nick Stokes: Milk. Gil Grissom: Cows drink water. They give milk. [Later] Nick Stokes: Hey, Catherine, say, “Silk, silk, silk.” Catherine Willows: Silk, silk, silk. Nick Stokes: What do cows drink? Catherine Willows: Water. Why? Nick Stokes: [after a disappointed pause] Never mind… ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [October 27, 2018, 1:55pm UTC](https://boards.straightdope.com/t/a-math-riddle-for-you-feel-free-to-submit-your-own/823205/117 "2018-10-27T13:55:33Z")
> [@notfrommensa](#): > > I think I would answer S-E-W for the first spelling quiz. Followed by “Faux” and “Know”.🙂 [Previous page](https://boards.straightdope.com/t/a-math-riddle-for-you-feel-free-to-submit-your-own/823205.md?page=5)