# another coin problem

#### recca02

Joined Apr 2, 2007
1,212
inspired by mr dave here is one more coin trick. although this is quiet a common
question for mba students lets see how tech freaks here go about it (in case if u know the answer already plz let others try first)
here is the problem.
there are ten coin minting machines.each coin manufactured must have an expected weight of 10 grams. but one of the machine develops a flaw and starts
developing 9 gram coins instead. in what way can one figure out which machine
is faulty by weighing just once.

#### Dave

Joined Nov 17, 2003
6,969
I've heard a similar one to this with water in 10 beakers, and you need to work out which beaker has less fluid in it than the others by measuring just once...I did know the answer, but time has a habit of wiping these things from your mind!

I'm sure it will be a discussion topic over the Easter dinner table

Dave

#### recca02

Joined Apr 2, 2007
1,212
I've heard a similar one to this with water in 10 beakers, and you need to work out which beaker has less fluid in it than the others by measuring just once...I did know the answer, but time has a habit of wiping these things from your mind!

I'm sure it will be a discussion topic over the Easter dinner table

Dave
may be thats what we call liquidity of funds --bad pun indeed.

#### hgmjr

Joined Jan 28, 2005
9,027
Removed my answer to give others a chance to come up with the solution.

hgmjr

#### recca02

Joined Apr 2, 2007
1,212
Exsponged my answer to give others a chance.....

hgmjr
cant believe someone wud give an answer that quick
you are indeed close to the answer, why didnt you apply the logic to all machines ?

#### hgmjr

Joined Jan 28, 2005
9,027
Exsponged my answer to give others a chance to solve the problem.....,

hgmjr

#### Papabravo

Joined Feb 24, 2006
20,391
Exsponged papabravo's correct answer to give others a chance to solve the puzzle....

#### recca02

Joined Apr 2, 2007
1,212
seems like this onewas a child's play for you ppl ,will bother u with something
rather harder later.
till then be afraid ..be very afraid. (--from 'fresh prince of bel air'--a great series)
for all others who havent yet thought of a valid answer please ignore the posts of mr papabravo and mr hgmjr.

#### Salgat

Joined Dec 23, 2006
218
Haha, Uncle Phil is awesome.

-------------------------
Very Good, Salgat. You have the right approach.

(Salgat's correct answer was exsponged to allow others to try.)

#### Dave

Joined Nov 17, 2003
6,969
I got this one last night in the bath!!

Its quite a clever technique that is very straightforward when you think of it. For those still trying to come up with the answer think how you can put coins from every machine into one pot such that their proportions are different, and hence differentiable. Is this giving too much away?

I will also try and remember the water in a beaker question that was a similar question (though not the same); something along the lines of shifting different proportions of fluid around without measuring - so its nothing like the conundrum in the OP!!

Dave

#### recca02

Joined Apr 2, 2007
1,212
thanks for 'ex sponging'
well since mr dave got the answer while taking a bath ,he must be "showered" with praises
cant wait for the beaker problem.
(actually i was thinking since volumes are different do we punch a hole at
some hieght and collect water for diff intervals or something, that of course if beakers arent transparent else we dont need to do anything ,do we?)

Haha, Uncle Phil is awesome.

-------------------------
Very Good, Salgat. You have the right approach.

(Salgat's correct answer was exsponged to allow others to try.)
good to see another uncle phil fan on the forum.

#### Dave

Joined Nov 17, 2003
6,969
cant wait for the beaker problem.
Some guy told me it in the pub, so my chances of catching up with the guy who told me are slim. I will just have to make up my own version!

Dave

#### recca02

Joined Apr 2, 2007
1,212
Some guy told me it in the pub, so my chances of catching up with the guy who told me are slim. I will just have to make up my own version!

Dave
well i guess that you will have to "coin" one
as far as laying this question to rest is concerned ,i think i wud leave this at the administrators' disposal or wud do so myself when the name of site changes to
"allabouteverything" or a next gen 3d dave game is launched or mr papabravo becomes sir grandpa bravo or mr beenthere turn mr been everywhere.
(no offence meant ,i apologise in advance if it hurts anybody )

#### Dave

Joined Nov 17, 2003
6,969
well i guess that you will have to "coin" one
as far as laying this question to rest is concerned ,i think i wud leave this at the administrators' disposal or wud do so myself when the name of site changes to
"allabouteverything" or a next gen 3d dave game is launched or mr papabravo becomes sir grandpa bravo or mr beenthere turn mr been everywhere.
(no offence meant ,i apologise in advance if it hurts anybody )
In the interests of those who didn't get it and as the OP, you can can have the privledge of putting this baby to bed.

Dave

#### recca02

Joined Apr 2, 2007
1,212
finally its time for coup de grace
the hint provided by mr dave is good enough though.
from "X" machine we must take "x" no of coins that are unique to that
machine.in this way we must take out coins from all machines. the mass
difference from expected mass of all genuine coins wud have some direct and distinguishable relation with the no of machine (no of coins taken from the machine to be precise).
eg; take one coin from first machine,2 from 2nd ..9 from ninth.(you dont need to take 10 coins from 10th one as method of exclusion can be used). now the
error value betn the expected and actual mass of coins is the machine no.
if no error is seen 10th machine is faulty.

#### Dave

Joined Nov 17, 2003
6,969
Interesting, I didn't think of the one exclusion but now see it possible.

Dave

#### Ron H

Joined Apr 14, 2005
7,014
I still don't get it. Kindly explain what you meant by "one weighing".

#### recca02

Joined Apr 2, 2007
1,212
I still don't get it. Kindly explain what you meant by "one weighing".
sorry abt my poor english,
its my second language u know.
by 'one weighing' i meant to find the faulty m/c in just single use of the weighing m/c.
i:e the use of weighing m/c more than once is forbidden, however any no of coins can be weighed in that single attempt.
(hope in case u r reading this thread for first time that u havent read my previous post )