[engineer question] coins

[martyr]

this isnt as hard as rope part 2, which smaug defeated.

the accepted answer for this is different than the one i came up with, and i know of another - there are options here.

also, this question is designed to generate dialogue with the interviewer, as though you were troubleshooting a software problem.



you have ten machines producing coins.
one machine is malfunctioning.
you may use a scale to weigh a mass only once.
determine which machine is malfunctioning.

[Baneblade]

Take the plates for the $100 bill and jump ship.

[Strygun]

what's the correct mass of a coin from a working machine?

[Strygun]

I've made another post because I bet you're reading the one i just made, and if I edited that, you wouldn't see it.

Are the coins from the working machines and the malfunctioning machine the same size? color? shape? density? combination of metal/metals?

[eXoSloth]

Go to the store and buy nine more scales...

[martyr]

the correct mass of a coin is 1g.

the coins from the machines are identical in all physical aspects except mass and density.

the malfunctioning machine is generating a bad alloy resulting in less mass but equal volume.

[Strygun]

what type of scale are we talking about here? a balancing scale, in that you place 2 coins on 2 sides and if they are good coins the scale will balance? or are we talking about a scale that measures mass?

[Indecisive]

Mass a coin, and compare it to the 9 other coins..how though. It would be friggen simple if you could weigh it multiple times

[Strygun]

are there other tools at our disposal? can we get other tools ourselves (within reason?)

[eXoSloth]

Put a coin from each machine in a bucket of water, the bad coin will float. (Maybe.)

[martyr]

triple-beam scale.

http://www.entrenet.com/~stevend/equip/triple_beam_balance_2.JPG

no other tools

[Strygun]

exo.. I dunno if we can use buckets of water :) but if we can, that was my idea first (I asked about density.)

[Baneblade]

By mass only once does that mean a single coin, or can you stretch it out to include all of them one at a time, just noting the difference.

[Strygun]

what is the mass of the bad coin?

[eXoSloth]

exo.. I dunno if we can use buckets of water :) but if we can, that was my idea first (I asked about density.)

It was our idea at the same time :p

[martyr]

each bad coin is exactly 1mg underweight == .99g

edit, out of curiosity, what would you do with a bucket of water, if you had one?

[Strygun]

depending on the alloy, one could theoretically take a sharp object and scratch each of the coins. if the coin does not scratch, it is the correct coin. if it scratches, more than likely it's a bad coin.

[EineBeBoP]

I'd take all the coins to your local vendor, see if he accepts them.
If he rejects one, thats the bad coin. If not, who gives a fuck as nobody will know the differance!

:p

[Jaged]

How much is a faulty coin's weight different from a normal coin's weight.

Assuming that all faulty coins weigh the same:

If you take 1 coin from the 1st machine, 2 from the 2nd, 3 from the 3rd, 4 from the 4th ect. Weigh the entire pile. If all the machines were working you should get 55g. Lets say that faulty coins all weigh 1/2g. If machine 1 was busted there would be 1 faulty coin. The total weight would be 54 1/2g. If machine 2 is busted there are 2 faulty coins in the mix. The total weight would be 54g. And so on....


Edit: I diddn't see maryter's post saying how much the faulty coins weigh. My solution still works for his faulty coin weight though.

[Indecisive]

...There is no solution O_o?

[Strygun]

what is the area of the coins? ( A = pi*(R*R) )

[Indecisive]

all pysical aspects are the same stry, volume would include the diameter of the coin, and the thickness.

[Jaged]

I just posted the answer guys, you can stop debating I belive.

[Strygun]

I know, I need to know for another math equation I'm working up

[Baneblade]

Oh that is easy then. Simply put them all one by one on the scale and keep adjusting the weights till you run across one that doesn't add up.

Don't take them off the scale, just stack em.

[Strygun]

Oh that is easy then. Simply put them all one by one on the scale and keep adjusting the weights till you run across one that doesn't add up.

Don't take them off the scale, just stack em.
that would be finding the weight of each coin, which you can't do.

[Indecisive]

you may use a scale to weigh a mass only once.
^

[Baneblade]

Fine, take 100 from each machine and lift em. It should be obvious which is lighter.

:shrug:

[Strygun]

Fine, take 100 from each machine and lift em. It should be obvious which is lighter.

:shrug:
doubt it, that would mean one pile is only 1g less than the others, and that's hard to detect.

[martyr]

depending on the alloy, one could theoretically take a sharp object and scratch each of the coins. if the coin does not scratch, it is the correct coin. if it scratches, more than likely it's a bad coin.

the rockwell measurements are identical

[Triggar]

Jaged wins :p

[martyr]

edit: yes. jaged wins.

[martyr]

How much is a faulty coin's weight different from a normal coin's weight.

Assuming that all faulty coins weigh the same:

If you take 1 coin from the 1st machine, 2 from the 2nd, 3 from the 3rd, 4 from the 4th ect. Weigh the entire pile. If all the machines were working you should get 55g. Lets say that faulty coins all weigh 1/2g. If machine 1 was busted there would be 1 faulty coin. The total weight would be 54 1/2g. If machine 2 is busted there are 2 faulty coins in the mix. The total weight would be 54g. And so on....


Edit: I diddn't see maryter's post saying how much the faulty coins weigh. My solution still works for his faulty coin weight though.


win!

and on to part two (you had to see this coming)

http://www.planetside-universe.com/forums/showthread.php?t=25983