12. Watson's Selection
You are shown a set of four cards placed on a table, each of which has a number on one side and a colored patch on the other side. The visible faces of the cards show 3, 8, red and brown.
Which card(s) must you turn over in order to test the truth of the proposition that if a card shows an even number on one face, then its opposite face is red?
Submitted by tartle · Added 1 January 2007
Hint: A response that identifies a card that need not be inverted, or that fails to identify a card that needs to be inverted, is incorrect. The original task dealt with numbers (even, odd) and letters (vowels, consonants).
Solution:
The correct response is to turn over the 8 card and the brown card.
The rule was "If the card shows an even number on one face, then its opposite face is red." Only a card with both an even number on one face and something other than red on the other face can invalidate this rule:
If the 3 card is red (or brown), that doesn't violate the rule. The rule makes no claims about odd numbers.
If the 8 card is not red, it violates the rule.
If the red card is odd (or even), that doesn't violate the rule. The red color is not exclusive to even numbers.
If the brown card is even, it violates the rule.
Comments (3)
I don't understand how to get the answer to puzzle #12. Please help me!
Can you please post the answer for problem 12? I can't get it right.
The hint in the string question contradicts the answer provided.
Add a Comment or Suggest an Answer