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Mathematical Puzzles
five cards

Fri Mar 04, 2011 5:04 am  by tartle

You are shown five cards lying on a table (so you can only see one side of each of the five cards). Each card has a single positive integer printed on each side of the card (they may or may not be repeated). The numbers that you can see are: 1, 2, 3, 4, and 5. How many cards must you turn over to completely verify the statement any card that has a 2 on one side has a 5 on the opposite side ?
Sun May 29, 2011 7:37 pm  by bds021

4, you need to know that the 1,3, and 4 all don't have a 2 on the other side. you also need to know that the 2 has a five on the other side. You don't need to flip the 5 because the converse of implication need not be true
Tue Jun 14, 2011 1:09 am  by snesdude

2 because it doesn't matter what's on the other side of 1,3, and 4. You need to know if there's a 5 on the other side of the 2 and a 2 on the other side of the five. Once you've figured that out you don't need to keep flipping because no matter what's on the other side of the 1,3, and 4 there most certainly isn't a 2 or a 5 on the side you can see.
Sun Jun 19, 2011 4:08 pm  by DiamondSoul

bds021 is correct.
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