You are shown a set of four cards placed on a table each of which has a number on one side and a coloured patch on the other side. The visible faces of the cards show 3, 8, red and brown. Which cards should you turn over in order to test the truth of the proposition:

If a card shows an even number, then its opposite face shows a primary colour?

For an explanation visit:

http://www.skepdic.com/refuge/ctlessons/lesson3.html

## 2 comments:

I found that reading the article, I never did get an answer to the problem. To prove that "If a card shows an even number, then its opposite face shows a primary colour?" I still think I only need to turn over the card with the 8 on it. The question is to prove if the card showing an even number has a primary colour on the back. Now, I could turn over the brown card to make sure an even number is not on the back. But you asked me to prove the statement, not disprove it. As to disprove it I would need to turn over 3 of the cards to make sure that an even number is not on the reverse side of the 3 and the brown cards, and also then to make sure that a primary colour is on the reverse of the 8 card. As for the red card, it doesn't matter what is on the reverse of this, as we didn't say primary colours are only limited to having even numbers on their reverse. Maybe primary colours can have any number on the back, but only even numbers have primary colours. I visited the skepdic.com site for an explanation, but found something way longer than what I just typed, and not sure there was an answer there.

I did mention that I would turn over the 3 card, even though there is a statement that each card has a number on one side and a colour on the reverse. If I am to question the "If a card shows an even number, then its opposite face shows a primary colour?" statement, why would that mean that all other statements about this are to be assumed as true?

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