Official Trivia/Riddles/Lateral Thinking Thread

[Regus]

At first I thought it had something to do with not using the scale at all and dunking a coin from each bag in water and seeing which one would float, but then I looked over the riddle and:

A man has ten bags of coins which all look alike. One of the bags is filled with fake coins. All the genuine coins weigh 10 grams and all the fake coins weight 20 grams each. Each of the ten bags contain 20 coins. Using a scale which measures in grams, how would he be able to determine which bag contains the fake coins with only one weighing?

I don't understand why this is even a riddle. We already know how much the real coins and fake coins weigh, and therefore, so does the man in the riddle. If he already has this knowledge, he could just place a single coin out of either bag and if it weighed more than 10 grams than it was fake, and since we already know that one bag contains all fakes while the other has the real ones, we can easily figure out which is which.

Also:

I mean, just by holding them.

This makes sense too, this was actually the first solution that came to mind.

[Calliander]

Okay... take one from the first bag, two from the second and so on.

Now, if they were all real, you'd have 550grams. Take 550 grams away from what you DO have and divide by 20. Then, it's that bag.

Also very close, but wrong.

[red_war_machine]

I'm not right.

I'm close though... ARGH!

[SearchManX]

What I was trying to say is that you were saying that the total of all the coins you removed was 90, which would be if you took one from only nine of the bags.

Yep, I realized it after RWM got it. Give the point to him, since I'd rather not worry about checking the board every 2 seconds.

Edit: Nevermind...

[red_war_machine]

I'm not right.

I'm close though... ARGH!

OKAY!

Take one from the first, two from the second, three from the third and so on.

Then... if they were all real, you'd have 550.

If 1 was fake, you'd have 550-10+20.
If 2 was fake, you'd have 550-20+40.

So, you take your answer, take 550 away from it and then divide the answer by 10. The fake bag is the one you took that many coins from.

!

Sly bastard.

...and proud of it. I do feel bad, though, as I'd never have got it without SMX's first try.

[Calliander]

I don't understand why this is even a riddle. We already know how much the real coins and fake coins weigh, and therefore, so does the man in the riddle. If he already has this knowledge, he could just place a single coin out of either bag and if it weighed more than 10 grams than it was fake, and since we already know that one bag contains all fakes while the other has the real ones, we can easily figure out which is which.

But that would require extreme luck in picking the fake coin or more than one weighing. And if you hold the bags, unless you pick the fake bag with the first two you pick up, you'll have to pick up more; more than one "weighing".

OKAY!

Take one from the first, two from the second, three from the third and so on.

Then... if they were all real, you'd have 550.

If 1 was fake, you'd have 550-10+20.
If 2 was fake, you'd have 550-20+40.

So, you take your answer, take 550 away from it and then divide the answer by 10.

That would be it. RWM gets the point! For example, if the sixth bag had the fake coins, there would be 610grams - one extra gram for each fake coin. (60 extra grams: 550 plus 60 equals 610.) With the divide by twenty method, you would get a wrong answer, hence the wrong bag: 60 extra grams divided by 20 is 3, not 6.

[MVS]

OKAY!

...and proud of it. I do feel bad, though, as I'd never have got it without Regus's first try.

well, gj to you and smx.

[Regus]

But that would require extreme luck in picking the fake coin or more than one weighing. And if you hold the bags, unless you pick the fake bag with the first two you pick up, you'll have to pick up more; more than one "weighing".

I don't understand what you mean. If he takes one coin out of a bag, doesn't matter which one, and puts it on the scale, if he sees that this single coin weighs 20 grams, then it's a fake, as is every other coin in that bag. If it weighed any less, than it would be real. He'd only have to do this once.

If you picked up both bags at the same time it would be as if you placed both bags on a scale, you'd only have to weigh them once.


Anyway, red_war_machine, please post a riddle that isn't based upon mathematics.

[MVS]

I'm out.

Pm me with any winners and I'll update them later.

[red_war_machine]

Okay, I'ma hit you with one of the oldest ones I know.

A man lives on the 18th floor of an apartment building. Every morning he comes out of his apartment, goes to the elevator and rides it to the ground floor. Every evening he comes back in, rides the elevator up to the EIGHTH floor and then takes the stairs up to the 18th. That is, unless there's someone else in the lift, in which case he'll always ride it straight up to the eighteenth. Why would he do this?