Airforce Riddle

Riddleman · Site Admin Joined: 18 May 2005 Posts: 595

They give you this riddle if you try out for the airforce. If you get it right, they give you a $30,000 bonus on your anual salery!

Ok, there are 8 dimonds. One is fake. The only way to tell if the dimond is fake is by the wieght. You have a scale in front of you, not one of the hanging ones, but one on a table. You can only wiegh them three times. How do you figure out which one is fake?

Note: If you put some on, and take one off, thats 2 wieghs, and if you take another off, thats three, so each time the number changes, its another wiegh.
Don't ask me cause i dont know, does anybody?

Guest · Posted: Tue Jan 31, 2006 11:59 pm Post subject:

see wich one scratches a window and the one that doesent is fake and if 2 weigh them

ory · Guest

get 4 diamonds and place them one the scale, if its wieght is not what 4 diamonds would normaly wiegh, then the fake one is one of those for. if it is then the fake is one of the four you did not wiegh. which ever batch of 4 it is in split it into 2 gruops of 2.
wiegh one group and use the same logic from the other time you wieghed them to find out which batch of 2 it is in, then wiegh those 2 and the one with the abnormal wieght is the fake[/quote]

shilly632 · New Member Joined: 31 Mar 2006 Posts: 8

place four on each side... which ever side weighs less has the fake....so take off the diamonds on the heavier side and split the remaining into 2 sets of 2... which ever sides weighs less has the fake... remove the heavier side agian and weigh the remaining 2 and which ever is lighter is the fake

Riddleman · Site Admin Joined: 18 May 2005 Posts: 595

i dont know the answer so any of you could be right

sweaterman · New Member Joined: 11 May 2006 Posts: 13

shilly is right.

GameMaster · Site Regular Joined: 20 Apr 2006 Posts: 55

I concur, shilly is right. That's classic [binary search/newton's method].

YaHo() · New Member Joined: 31 May 2006 Posts: 12

i've heard this before, and its REALLY HARD...it was a noble prize winners questioin and its hard because, u dont know if the fake is heavier or lighter. the answer is long, u best to search google for it.

Ipshwitz · New Member Joined: 23 May 2006 Posts: 21

It's not a hanging scale so you can't place 4 diamonds on one side and 4 diamonds on another side.

Again, you don't know which is the heavier of the two. Granted, compressed carbon should weigh more than glass, plastic, and most other types of clear materials, but I doubt that is a given (as it's not stated in the riddle).

I'm assuming you can only compare weights by placing an item on the scale, taking it off, placing another item on the scale and comparing the two weights. Which of course is 2 out of three measurments.

--- Let me think on this one (grabbing calculator)

memyself&i · New Member Joined: 09 Jun 2006 Posts: 10

well actually u can tell which one is real by which one reflects more color... if you can see a rainbow of color in one then that is the fake one

o ya & glass can scratch glass so if the fake on is glass then u still wouldnt kno

Blaze · Posted: Fri Jun 23, 2006 3:18 pm Post subject:

Heres another way to solve it. Mohs' hardness scale shows that diamonds are the hardest minerals known and if something is harder then another thing, e.g. talc and diamond, the harder thing will scratch or take away bits of the less harder thing, so, the fake diamond wont be able to scracth any of the real diamonds

BoloMark33 · New Member Joined: 25 Jul 2006 Posts: 5

Ok, I've got part of the logic for solving this one worked out... unfortunately, it's the easy part Smile

...

I think you should divide them into two groups of 3 and one group of 2.

1) Weigh a group of 3 and record its weight. That is weight A.
2) Weigh the other group of 3 and record its weight. That is weight B.

Now, if A = B, then the fake diamond is among the remaining two. To figure out which one, just place one of them on the scale (weight C). If it weights 1/3*A (or *B), then its a real diamond and the one not on the scale is fake. Otherwise, if its not a nice fraction of the previous weights, you have placed the fake diamond on the scale.

You have only weighed them three times (A, B, C)

Now, if A not= B, then... still thinking about that one.

BoloMark33 · New Member Joined: 25 Jul 2006 Posts: 5

Hmm... I was only able to get it down to 4 weighings, and even then I'm not sure of my logic.

I threw the question over to another riddle-solving forum and it appears that the question as stated may be impossible to fully answer. For an explanation, go here:

http://www.able2know.com/forums/viewtopic.php?t=79785

ff9pro · New Member Joined: 17 Aug 2006 Posts: 1

Well, i suppose by weigh they mean using the scales.
use your hands to weigh...

Kazekage-Gaara · New Member Joined: 24 Dec 2006 Posts: 1

Easy. If one is a glass fake, throw them on the ground. The fake will shatter. If plastic, it will obviously weigh less, and almost any of the above methods will work. You could also randomly guess, which might work. A final method is to eat them, and the one that doesn't break your teeth is fake.

Riddleman · Site Admin Joined: 18 May 2005 Posts: 595