You are given eight coins and told that one of them is counterfeit. The counterfeit one is slightly heavier than the other seven. Otherwise, the coins look identical. Using a simple balance scale, how can you determine which coin is counterfeit using the scale only twice?
First weigh three coins against three others. If the weights are equal, weigh the remaining two against each other. The heavier one is the counterfeit. If one of the groups of three is heavier, weigh two of those coins against each other. If one is heavier, it’s the counterfeit. If they’re equal weight, the third coin is the counterfeit.
In 2012 a class was divided into 2 groups. Their assignment was to find the names of at least 3 children who were born on the same day from 5 different months of 2011. These were the results:
Group 1
August 20: Oliver, William, and Adam.
January 3: John, Alice, and Ken.
September 7: Bruce, Shane, and Peter.
June 11: April, Patrick, and Bobby.
July 19: Trent, Julie, and Charles.
Group 2
March 1: Karl, Willie, and Patty.
February 29: Blake, Kobe, and Wayne.
December 24: Kyle, Chad, and Zoe.
May 12: Matthew, Manny, and Adrian.
November 20: Greg, Fiona, and Elizabeth.
The members of group 2 got an F on the assignment. Why?
This problem can be solved by preschool children in five to ten minutes, by programmers in an hour and by people with higher education…well, check it yourself.