Interview Question for `Systems Validation Engineer` at `Intel Corporation`:
`May 06, 2010`

`You have 8 pennies, 7 weight the same, one weighs less. you also have a judges scale. Find the one that weighs less in less than 3 steps.`

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Answers & Comments (`13`)

4 of 7 people found this helpful

May 06, 2010

by Interview Candidate:

Set two pennies aside, put 3 on each scale and if they weight the same the lighter one is one of the other two, if not it is one of the three on the scale, repeat for the two or three remaining.

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14 of 17 people found this helpful

Sep 26, 2010

by Neil:

A better answer:

Split the 8 pennies into 3 groups of 3,3,2 pennies. Weight the first two groups of 3 pennies each.

Case 1) - They weight the same. Therefore, take third group of 2 pennies and find the lighter coin.

Case 2) Group 1 weights more than Group 2. Take group 2 (3 pennies) and pick any 2 out of 3. If they weight the same, then the third penny is lighter. Answer is trivial if they don't weight the same.

It works for any scenario. Always split the groups in 3.

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5 of 12 people found this helpful

Dec 30, 2010

by Dave:

It's a lot simpler than you'd think
Divide the current number of coins by two and put them on both sides of the scale, remove the side that weighs more, and divide the lighter side in half, repeat
so you start with 8:
|||| |||| (4/4)
|| || (2/2)
| | (1/1)
Three steps, simple, logical, and and can get the right answer each time.

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5 of 5 people found this helpful

Dec 30, 2010

by Tim:

Neil has it right. Dave - the question says less than three steps.

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0 of 1 people found this helpful

Jan 06, 2011

by samantha:

Actually, Neil is still correct. The question does not say less than three steps, it says three steps. You must have misread it Tim. "Find the penny that weighs less in three steps." not less than, it's less in... And they both seem as though they would work and be relatively simple to perform, explain, and understand.

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3 of 3 people found this helpful

Jan 09, 2011

by Matthew:

I Can do it in one step: Look at the year on the pennies.

If 7 pennies weigh the same it means that all 7 were manufactured either before or after 1982, when the mint changed the composition of the coin from solid copper to copper with a zinc core. the one that weighs differently was minted on the opposite side of 1982 as the 7 other pennies.

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1 of 1 people found this helpful

Jan 09, 2011

by Matthew:

To clarify and adhere to the the question better. the 7 pennies that weigh the same are pre 1982 and the one that weights less is post 1982 because zinc weighs less than copper.

but like I said. the one step is to simply look at the date.

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0 of 1 people found this helpful

Jan 11, 2011

by Adam:

Step One:
Place two pennies aside and weigh 3 vs 3.

Step Two:
If they are equal then weigh the two other pennies.

If a group of three pennies contains the lighter penny then cut one of them in half. Weigh the two groups of one and a half pennies. If its equal then the cut penny is the lightest, otherwise it is the whole penny on the lighter side.

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