Est. 1998 — 27 years of woodworking knowledge
Friday Puzzler -- Card Logic
Alex Y
There are 4 cards lying flat on a table. The four sides of the cards that are showing are:
One solid blue card
One solid green card
One card with the number 8
One card with the number 5
I claim that for these four cards, every card with an even number on one side is solid blue on the other side.
How many and which cards do you need to turn over to test the veracity of my statement?
Re: Friday Puzzler -- Card Logic
Larry Barrett
Two. The card with the 8 to verify it is blue, and the green card to verify that it is not even.
Close
Alex Y
Yours is the correct solution to a very similar problem, which you probably subconsciously read this as.
Re: Close
Henry Higginbotham
Do the cards necessarily have a color on one side and a number on the other?
Bingo!
Alex Y
That's the condition that is part of the original puzzle, and that I left off (intentionally, but often left off in error). This is called the Wason selection task https://en.m.wikipedia.org/wiki/Wason_selection_task
Larry's answer is correct given that precondition.
What is interesting about this logic puzzle is that done in the abstract with even and odd numbers and two different colors, it can be pretty hard. But a real life application is trivial:
You want to determine if a law preventing persons under age 18 from drinking alcohol beverages is being complied with. You walk into a room and see two individuals who look to be in their late teens, one drinking a beer and one drinking a Coke. Two more people are drinking from opaque glasses, so you can't immediately tell what they are drinking. One of these individuals is clearly a young teen and the other is in his 40's. No one would have any problem determining which ids and which beverages to check. While no one will feel the need to see if the 40-yr-old is drinking beer, it is surprising how many people feel the need to check what is on the other side of the blue card.