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9.15.2005

Another brainteaser 

Given 100 fair coins on a table, 10 of which are heads-up and 90 of which are tails-up, create two sets of coins such that each has an equal number of heads-up coins. The problem is that you are blindfolded and are wearing gloves, so you can't tell if a given coin is heads-up or tails-up. Every coin must be in exactly one of the two sets (i.e., two empty sets doesn't count).

Highlight the text below for a solution.


Separate the coins into a set of 90 and a set of 10. Suppose that there are a heads-up coins in the set of 10. There are then 10-a heads-up coins in the set of 90.

Flip all the coins in the set of 10. There are now 10-a heads-up coins in the set of 10.

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