Blindfolded coin puzzle
November 17th, 2024 at 6:20:50 PM
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I came across an interesting puzzle/riddle used in an interview recently.
You are sitting at a table blindfolded. There are 100 quarters on the table top. 10 are heads up / 90 are tails up.
Your task is to separate the coins into two groups. The two groups of coins need to have an equal number of heads up coins in each.
You can handle the coins in anyway you wish, however you are wearing gloves so you cannot distinguish heads or tails.
You must use all 100 quarters.
Discuss…
You are sitting at a table blindfolded. There are 100 quarters on the table top. 10 are heads up / 90 are tails up.
Your task is to separate the coins into two groups. The two groups of coins need to have an equal number of heads up coins in each.
You can handle the coins in anyway you wish, however you are wearing gloves so you cannot distinguish heads or tails.
You must use all 100 quarters.
Discuss…
Son you ain’t paying attention
I’m cutting you but you ain’t bleeding
- Foghorn Leghorn
November 17th, 2024 at 6:41:20 PM
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Quote: linksjunkieI came across an interesting puzzle/riddle used in an interview recently.
You are sitting at a table blindfolded. There are 100 quarters on the table top. 10 are heads up / 90 are tails up.
Your task is to separate the coins into two groups. The two groups of coins need to have an equal number of heads up coins in each.
You can handle the coins in anyway you wish, however you are wearing gloves so you cannot distinguish heads or tails.
You must use all 100 quarters.
Discuss…
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I saw this too - I think it was posted as a "Harvard entrance exam" question on Facebook.
However, that version left out an important part.
The original problem said that you could separate the coins, and not that you "could handle them in any way." Given that condition, it would be impossible.
The answer is to take any 10 coins and turn them over.
Let H be the number of heads in the 90 coins you did not take; there would be 10-H heads and H tails in the 10 that you did.
When you turn the 10 over, there are now H heads in each pile.
November 17th, 2024 at 7:00:28 PM
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Correct. I made sure I included “handle” as I agree with you assessment on the wording.
Son you ain’t paying attention
I’m cutting you but you ain’t bleeding
- Foghorn Leghorn
November 17th, 2024 at 7:22:03 PM
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I asked that puzzle in my last newsletter.
"For with much wisdom comes much sorrow." -- Ecclesiastes 1:18 (NIV)
November 17th, 2024 at 9:46:23 PM
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I am just dirty enough to do this. Do I get extra credit for telling which ones are bicentennial?
Rub each face of the coin against my teeth. The smoother profile of Washington on the obverse will be easily distinguishable from the rough surface of the various designs on the reverse. Rubbing the edge of one coin against the faces of the other should produce an audibly different result on each side too.
November 18th, 2024 at 12:53:06 AM
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Quote: AutomaticMonkeyI am just dirty enough to do this. Do I get extra credit for telling which ones are bicentennial?
Rub each face of the coin against my teeth. The smoother profile of Washington on the obverse will be easily distinguishable from the rough surface of the various designs on the reverse. Rubbing the edge of one coin against the faces of the other should produce an audibly different result on each side too.
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Practically, I think the proctor is going to shove a mitten in your mouth as a reward for original thinking.
May the cards fall in your favor.
