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[Reason]

[SOLVED] #054 - Orange - 9 Ball

[appleTRON]

I think it's 3 weighings ...

Say you group balls 1 - 4 and balls 5 - 8 and weighed them against each other. One weighing. (If they weighed evenly, you could then weigh the 9 ball against one of the others, and it would be the 9 ball).

Say the 1st group weighed lighter. Then you could weigh balls 1 & 2 against 3 & 4. Two weighings.

Say 1 & 2 were lighter. Then you could weigh them against each other and determine which was the lighter one. Three weighings.

So, you could do it in two, but three proves it.

Just a guess, though.

EDIT: Actually, I think my logic is a bit flawed, but I'm on the right track ... any help?

PS - man, I love me that Caine Johannson. He's so hot right now.

[toongoon]

God call. I believe you have the right idea but I would split them into 3 groups of 3 (aaah 333!)

A similar puzzle with solution is listed here (I don't feel the energy to type it all out. )

[nmarciano]

I actually think you could do it in 2 weighings.
You take balls 1,2,3 vs 4,5,6
(1) if =, you take ball 7 vs 8
(a) if =, ball 9 is the light one
(b) if not =, its ball 7 or 8
(2) if not =, you note which side is lighter (for example, balls 1,2,3) and measure 2 of those balls against each other
(a) if =, its ball 3
(b) if not =, its ball 1 or 2

2 weighings and you've found the light ball

[crumple]

Maybe it depends how we read the question.

WHAT WAS THE MINIMUM NUMBER OF WEIGHINGS OF THE NINE BALL POOLS I NEEDED TO TAKE TO FIND OUT WHICH WAS LIGHTER THAN THE OTHERS?

It seems implicit in the question that one ball is lighter than the others. It says "find out which was lighter", not "find out whether one was lighter".

If that reading is correct, it could be done in one weighing (the minimum number). You weigh balls 1-4 against 5-8. If they balance, then 9 must be the lighter one.

Or am I being pedantic here?

[SteveC]

Slight flaw in logic, if they don't match, you're still not sure which of four balls it is.

[sixsidedsquare]

I've heard this sort of riddle before and I'm going with 2.

[JebJoya]

nmarciano is correct, here's just a simplified version to check the logic:

1) WEIGH ( 123 ) AGAINST ( 456 )
If equal go to step ( 2 )
If ( 123 ) lighter go to step ( 3 )
If ( 456 ) lighter go to step ( 4 )

2) WEIGH ( 7 ) AGAINST ( 8 )
If equal, BALL 9 is the LIGHT one
If ( 7 ) lighter, BALL 7 is the LIGHT one
If ( 8 ) lighter, BALL 8 is the LIGHT one

3) WEIGH ( 1 ) AGAINST ( 2 )
If equal, BALL 3 is the LIGHT one
If ( 1 ) lighter, BALL 1 is the LIGHT one
If ( 2 ) lighter, BALL 2 is the LIGHT one

4) WEIGH ( 4 ) AGAINST ( 5 )
If equal, BALL 6 is the LIGHT one
If ( 4 ) lighter, BALL 4 is the LIGHT one
If ( 5 ) lighter, BALL 5 is the LIGHT one


Hope that clears everything up for people

Jeb

[appleTRON]

Okay from now on, I'm just letting Jeb do the math ... er, logic. Same difference.

[Seej]

This is another puzzle that's similar to one of the Microsoft Interview Questions.

[Rob_Riv]

yeah agreed answer's 2 then

[oliverkeers13]

Nice avatar bump/post whoring there rob.

[Riiick]

Never having seen weighing scales in a pub, are we sure that they don't give a weight (sprunt/electric scales) rather than letting you compare the weights of the balls (balance type).

This would need either 2 weighs on a balance type scale or 4 weighs on a sprung/electric type scale

Also the suggestions is that you have to prove which ball is lightest, although the question is ambiguous.

So to prove the ball is lighter would require 3 or 5 weighings depending on the type of scales.

[_ender]

every puzzle i have seen with scales assumes the balancing kind

[tanner]

the one where its 1 of a, 2 of b, 3 of c ect uses a spring balance