# Brain Teasers **URL:** \ **Category:** The Game Room\ **Created:** [July 26, 2010, 4:05pm UTC](https://boards.straightdope.com/t/brain-teasers/548065 "2010-07-26T16:05:31Z")\ **Posts on this page:** 16\ **Page:** 3
**Author:** ![Baracus](https://avatars.discourse-cdn.com/v4/letter/b/2bfe46/32.png) [@Baracus](https://boards.straightdope.com/u/Baracus)\ **Post date:** [August 11, 2010, 7:37pm UTC](https://boards.straightdope.com/t/brain-teasers/548065/41 "2010-08-11T19:37:11Z")
> [@glowacks](#): > > For 5, I don’t like the “wait for night” answer, because that assumes that Tom _can_ wait for night. Tom is already established as being in a fantasy world if there’s a dragon behind one door, so the sun might never set - suppose it’s on a planet with with rotation equal to revolution time. It’s never said that Tom knows what’s behind each door, or that he’s able to tell whether the sun is down. You need to make a number of assumptions that are not obvious in order for “wait for night” to make sense. Well, it seems like if we don’t assume something (Tom has a flame-retardant suit, the dragon is a heavy sleeper, Tom knows a wizard) then Tom is pretty much stuck. Since the first words of the teaser are “one day” it seems like a pretty minor assumption that Tom’s fantasy world still has night and day. Anyway, as far as knowing what is behind which door. Tom feels each door. Assuming (again) that the dragon isn’t blasting away indiscriminately, the glass room door should be warmer than the dragon door. When the door cools down, Tom concludes that the sun has set and the door is safe to open. ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [August 11, 2010, 9:20pm UTC](https://boards.straightdope.com/t/brain-teasers/548065/42 "2010-08-11T21:20:19Z")
> [@Chronos](#): > > Or an implausibly steep hill or pit. Yep. Or a ridge, a gully or both. The tetrahedron is the key - implementation is left as an exercise for the student. ---
**Author:** ![Indian](https://avatars.discourse-cdn.com/v4/letter/i/c0e974/32.png) [@Indian](https://boards.straightdope.com/u/Indian)\ **Post date:** [August 18, 2010, 6:46am UTC](https://boards.straightdope.com/t/brain-teasers/548065/43 "2010-08-18T06:46:54Z")
Bumping,hoping for some more of these .🙂 ---
**Author:** ![Ximenean](https://avatars.discourse-cdn.com/v4/letter/x/aca169/32.png) [@Ximenean](https://boards.straightdope.com/u/Ximenean)\ **Post date:** [August 19, 2010, 3:31pm UTC](https://boards.straightdope.com/t/brain-teasers/548065/44 "2010-08-19T15:31:27Z")
I saw a nice on Futility Closet the other day: You have 100lb of potatoes that are 99% water, by weight (OK, they’re strange potatoes). You let them dehydrate until they’re 98% water. How much do they weigh now? ---
**Author:** ![Hoopy\_Frood](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hoopy_frood/32/8452_2.png) [@Hoopy\_Frood](https://boards.straightdope.com/u/Hoopy_Frood)\ **Post date:** [August 19, 2010, 4:01pm UTC](https://boards.straightdope.com/t/brain-teasers/548065/45 "2010-08-19T16:01:51Z")
I saw that one and solved it. It’s counterintuitive, isn’t it? Unless you actually look at the math, and then it makes sense. ---
**Author:** ![Tom\_Scud](https://avatars.discourse-cdn.com/v4/letter/t/f08c70/32.png) [@Tom\_Scud](https://boards.straightdope.com/u/Tom_Scud)\ **Post date:** [August 19, 2010, 4:05pm UTC](https://boards.straightdope.com/t/brain-teasers/548065/46 "2010-08-19T16:05:45Z")
> [@Ximenean](#): > > I saw a nice on Futility Closet the other day: > > You have 100lb of potatoes that are 99% water, by weight (OK, they’re strange potatoes). You let them dehydrate until they’re 98% water. How much do they weigh now? [spoiler]Originally, they have 99 pounds of water to 1 pound of non-water-potato-substance. Subsequently, they are 98% water, or 49/50; they still have 1 pound of NWPS, so therefore they have 49 pounds of water, and weigh a total of 50 pounds.[/spoiler] ---
**Author:** ![Arnold\_Winkelried](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@Arnold\_Winkelried](https://boards.straightdope.com/u/Arnold_Winkelried)\ **Post date:** [August 20, 2010, 12:42am UTC](https://boards.straightdope.com/t/brain-teasers/548065/47 "2010-08-20T00:42:16Z")
I think I have new answers to a couple of these: > [@KennerTheGreat](#): > > 1. After a power surge disrupted all the electronic devices in Brian’s house, he had to go over to Amy’s to find out what time it was. After she told him, Brian walked home and set all of his clocks. Even though Brian didn’t know how long it had taken him to get home, he was able to set his clocks to exactly the right time. How? Brian didn’t know how long it took him to go home from Amy’s house, but he knew how long it took to _go to_ Amy’s house. When he left, one of his clocks had the time 9:00 (because it restarted, or because he set it to 9:00). When he got to Amy’s house, which took him 10 minutes, he knew his clock would be saying 9:10. But Amy told him it was 11:30. He knew that when he got back, he would have to move that clock ahead by 2 hours and 20 minutes, and then make all the other clocks match the correct clock. > [@](#): > > 1. One day, Tom found himself trapped in a room with two doors. Behind the first door was a room made of thick glass that would magnify the sun’s heat to fry whoever went in. Behind the second door was a fire-breathing dragon that would do the same. How did Tom escape unscathed? Through a window. ---
**Author:** ![Arnold\_Winkelried](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@Arnold\_Winkelried](https://boards.straightdope.com/u/Arnold_Winkelried)\ **Post date:** [August 20, 2010, 12:49am UTC](https://boards.straightdope.com/t/brain-teasers/548065/48 "2010-08-20T00:49:38Z")
By the way, I’m pretty sure I read this one at the SDMB, but my memory could be faulty. Which weighs more: an ounce of gold, or an ounce of feathers? A: An ounce of gold. Gold is measured in troy ounces which are 31 grams. Feathers would be measured in avoirdupois (“regular”) ounces which are 28 grams. ---
**Author:** ![Hoopy\_Frood](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hoopy_frood/32/8452_2.png) [@Hoopy\_Frood](https://boards.straightdope.com/u/Hoopy_Frood)\ **Post date:** [August 20, 2010, 6:19pm UTC](https://boards.straightdope.com/t/brain-teasers/548065/49 "2010-08-20T18:19:10Z")
> [@Arnold\_Winkelried](#): > > By the way, I’m pretty sure I read this one at the SDMB, but my memory could be faulty. > > Which weighs more: an ounce of gold, or an ounce of feathers? > > A: > > An ounce of gold. Gold is measured in troy ounces which are 31 grams. Feathers would be measured in avoirdupois (“regular”) ounces which are 28 grams. Oddly enough though, a pound of Gold will weigh less, because the troy pound only has 12 troy ounces. ---
**Author:** ![Waterman](https://avatars.discourse-cdn.com/v4/letter/w/b2d939/32.png) [@Waterman](https://boards.straightdope.com/u/Waterman)\ **Post date:** [August 20, 2010, 10:39pm UTC](https://boards.straightdope.com/t/brain-teasers/548065/50 "2010-08-20T22:39:32Z")
> [@Tom\_Scud](#): > > > [@Ximenean](#): > > > > I saw a nice on Futility Closet the other day: > > > > You have 100lb of potatoes that are 99% water, by weight (OK, they’re strange potatoes). You let them dehydrate until they’re 98% water. How much do they weigh now? > > [spoiler]Originally, they have 99 pounds of water to 1 pound of non-water-potato-substance. > > Subsequently, they are 98% water, or 49/50; they still have 1 pound of NWPS, so therefore they have 49 pounds of water, and weigh a total of 50 pounds.[/spoiler] They would weigh 9900 pounds since you started with 100 pounds of potatoes! (100/0.01)\*0.98 [this is the final weight of water] + 100 [this is the weight of potatoes] = 9900 ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 20, 2010, 11:14pm UTC](https://boards.straightdope.com/t/brain-teasers/548065/51 "2010-08-20T23:14:03Z")
**Waterman** , I think you misunderstood the problem. You start with 100 pounds, then you lose weight due to drying. There’s not separate water and potatoes; the water is part of the potatoes. ---
**Author:** ![Waterman](https://avatars.discourse-cdn.com/v4/letter/w/b2d939/32.png) [@Waterman](https://boards.straightdope.com/u/Waterman)\ **Post date:** [August 23, 2010, 6:41am UTC](https://boards.straightdope.com/t/brain-teasers/548065/52 "2010-08-23T06:41:23Z")
> [@Chronos](#): > > **Waterman** , I think you misunderstood the problem. You start with 100 pounds, then you lose weight due to drying. There’s not separate water and potatoes; the water is part of the potatoes. After rereading the problem you are correct. As \*\*\*Tom Scud \*\*\*said, the final weight is 50 pounds after losing 50 pounds of water. ---
**Author:** ![kaylasdad99](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kaylasdad99/32/3398_2.png) [@kaylasdad99](https://boards.straightdope.com/u/kaylasdad99)\ **Post date:** [August 25, 2010, 5:09am UTC](https://boards.straightdope.com/t/brain-teasers/548065/53 "2010-08-25T05:09:29Z")
Actually, after losing 49 pounds of water. ---
**Author:** ![Ximenean](https://avatars.discourse-cdn.com/v4/letter/x/aca169/32.png) [@Ximenean](https://boards.straightdope.com/u/Ximenean)\ **Post date:** [August 26, 2010, 1:25am UTC](https://boards.straightdope.com/t/brain-teasers/548065/54 "2010-08-26T01:25:43Z")
Those of you familiar with mediaeval history may have noticed that Louis VII became king of France in 1137, and that Edward I ascended the English throne in 1272, both of which years have digits that add up to twelve. As it happens, there are three other historical monarchs whose reigns also began in years whose digits add up to twelve, one of whom ascended the throne in the year 1155. Let’s call them Queen N, King S, and King R. The queen’s reign began 45 years before King S, and his reign began 135 years before R’s. In which years did each of their reigns begin? (adapted from a puzzle by D. St. P. Barnard) ---
**Author:** ![Indistinguishable](https://avatars.discourse-cdn.com/v4/letter/i/90ced4/32.png) [@Indistinguishable](https://boards.straightdope.com/u/Indistinguishable)\ **Post date:** [August 26, 2010, 1:37am UTC](https://boards.straightdope.com/t/brain-teasers/548065/55 "2010-08-26T01:37:46Z")
1335, 1290, and 1155. B.C. (Who are the monarchs?) ---
**Author:** ![Ximenean](https://avatars.discourse-cdn.com/v4/letter/x/aca169/32.png) [@Ximenean](https://boards.straightdope.com/u/Ximenean)\ **Post date:** [August 26, 2010, 1:45am UTC](https://boards.straightdope.com/t/brain-teasers/548065/56 "2010-08-26T01:45:34Z")
Do you have any idea how long it took me to find actual monarchs for whom that worked? 😃 The monarchs really are N., S., and R. I leave their identities as an exercise for the reader. I was always tickled by that puzzle in its original form, but the dates didn’t match up with what historians now think. The version I presented here at least matches what Wikipedia says. [Previous page](https://boards.straightdope.com/t/brain-teasers/548065.md?page=2)