Another puzzle from clop

[clop]

Greetings from Adelaide.

Here's another puzzle. I got the answer to this one wrong so hopefully you will get the answer wrong too. Please don't cheat and look at other people's answers! It will be interesting to see how many people get the answer right first time.

There are four cards lying on the table in front of you. Each card has a letter on one side and a number on the other. The cards look like this

D F 3 7

You are required to verify the rule that "if a card has a D written on one side, it will have a 3 written on the other."

How many cards do you need to turn over to satisfy yourself that this rule is true? Please specify which card or cards, in your reply below.

clop

[Jeff Root]

As long as I have the opportunity to be first, I might as well...

I haven't looked at ANYTHING yet except the question, and I
don't recall ever seeing this particular question before.

Three cards must be turned: The D card has to be turned to be
sure it has a 3 on the back. The F and 7 cards have to be turned
to be sure they do not have D on the back. The 3 card does not
need to be turned because it is ok nomatter what is on the back.

-- Jeff, in Minneapolis

[Chuck]

The F card can't have a D on the back because each card has a letter and a number.

[Chuck]

I have to turn the D card to see if there's a 3 on the back. If there's no 3 then I'm done because I know that the rule is false. If there is a 3 then I have to turn the 7 card to make sure there's no D on the back. So 1 card for sure and maybe 2. The answer is 1½ cards.

[Peter Canuck]

Wouldn't it be 2 cards.
You would need to turn over the D to see if there is a 3 and hte 3 to see if there is a D. There should be no assumption that other cards with letters can or can not also have a 3 on the bacl.

[Jeff Root]

Ah! I didn't read carefully enough! I only read the first part of the
instructions once, then mis-remembered it.

-- Jeff, in Minneapolis

[Tog]

I said 3.
The D to verify a 3
The 3 to verify a D
The 7 to verify no D.
THe rule stated that a card with a D must have a three on the other side, so even if there was a 3 on the F card, it wouldn't affect the rule.

Looking at the question again though, I think the 3 wouldn't matter either, as it could ave any letter and still not violate the rule.

It's too late, but I thin I should have gone with 2.:(

[antoniseb]

I said 2, the D and the 7.
Assuming that the rule only applies to this set of four cards, then for it to be true, the 'D' must have a '3' on the other side, and the '7' cannot have a 'D' on the other side. The currently exposed '3' can have anything on the otherside, because the rule didn't say anything about one-to-one relationships between letters and numbers. Likewise the rule says nothing about the 'F'.

[turbo-1]

Turn the D to ensure that there is a 3 on the other side. Turn the 7 to make sure that there is no D on the other side.

[SeanF]

Without looking at anybody's answers, I picked 2 - the "D" card and the "7" card need to be turned over. If the "D" card has something other than a 3 on the other side, or if the "7" card has a "D" on the other side, the theory is disproven. It doesn't matter what's on back of the "3" or the "F" cards.

[Eroica]

Without looking at anyone else's answer I said 2. My reasons, it turns out, are the same as those already stated by astoniseb and SeanF.

I assumed from the wording of the question that the rule is true.

As clop got it wrong, I'm a bit worried that the correct answer is 1, though I can't see how it could be (unless the rule doesn't hold and the D has something other than a 3 on the other side, in which case there is no need to go any further).

[ToSeek]

D and 7, as has been explained already.

[ritchyrev]

Yep I say the same D and 7
Took me 2 seconds.
I have done these tests before.
You have to read the rules through a few times.
Heres a link to more of the same.
Give them a try and then check out the stats.
Without a lie I scored 100% on the main and all additional tests.
Each one took me a while though.

http://www.philosophersnet.com/games/logic_task.htm

[TheBlackCat]

I came up with D and/or 7 also.

[Eroica]

I came up with D and/or 7 also.
Also? Who else came up with "D and/or 7"?

[gwiz]

Me too. It's "and" if your first pick confirms it, if not you don't need to pick the second.

[Eroica]

It's "and" if your first pick confirms it, if not you don't need to pick the second.
Oh, right. I had forgotten about that.:o

[SeanF]

Me too. It's "and" if your first pick confirms it, if not you don't need to pick the second.
Ah, but the question was:


How many cards do you need to turn over to satisfy yourself that this rule is true?
The first pick can't confirm it. The first pick could deny it, in which case there's no point in turning over the second card, but both are necessary to confirm it.

;)

[jfribrg]

I voted 3: D, F, and 7

D to confirm that there is a 3 on the other side.

F and 7 to confirm that there isnt a D on the other side.

the 3 card is irrelevant. Since the assertion we are testing does not say that the 3 only appears on a D card, whatever is on the other side cannot change our conclusion.

[pghnative]

I voted 3: D, F, and 7

D to confirm that there is a 3 on the other side.

F and 7 to confirm that there isnt a D on the other side.

the 3 card is irrelevant. Since the assertion we are testing does not say that the 3 only appears on a D card, whatever is on the other side cannot change our conclusion.
I did exactly the same, but we both missed that the rules state that each card has one number and one letter, so therefore "F" need not be checked.

[Sam5]

I would say 2, the D card and the 7 card.

If the D card doesn’t have the 3, you’ve disproved the rule. If it does have a 3 you’ll still have to turn over the 7 card. If the 7 has a D you’ve disproved the rule. If the D card does have a 3, you haven’t proved the rule, since the 7 card could also have a D on the other side. What the 3 and F cards have on the other side is not important.

[Inferno]

2 - the D and 7.

[i only ponder]

I think the answer's 2.
-you need to turn over the D to see of there's a 3 on the back
-you turn over the 7 to be sure there isn't a D on the back

[Eroica]

I think the answer's 2.
-you need to turn over the D to see of there's a 3 on the back
-you turn over the 7 to be sure there isn't a D on the back
:whistle:

Methinks someone has been editing their messages....

[GDwarf]

:whistle:

Methinks someone has been editing their messages....
Nope, vBulliten has a little message at the bottom of a post telling you if and when it's been edited.

Edit: You should now see it below this message.


Edit: Or that could be disabled by the mods for some reason... :confused:

[clop]

Nope, vBulliten has a little message at the bottom of a post telling you if and when it's been edited.

Edit: You should now see it below this message.


Edit: Or that could be disabled by the mods for some reason... :confused:

I think you can edit posts without the edit comment being appended as long as you edit it before anyone has viewed it.

clop

[Eroica]

I think you can edit posts without the edit comment being appended as long as you edit it before anyone has viewed it.

clop Correct (well, before someone else posts another message, actually), and that's exactly what i_only_ponder did. I know because I received an email notification of his post which quoted his message thus:


Dear Eroica,

i only ponder has just replied to a thread you have subscribed to entitled - Another puzzle from clop - in the Fun-n-Games forum of Bad Astronomy and Universe Today Forum.

This thread is located at:
http://www.bautforum.com/showthread.php?t=40443&goto=newpost

Here is the message that has just been posted:
***************
I think the answer's 3.
-you need to turn over the D to see of there's a 3 on the back
-you turn over the 3 to see of there's a D on the back
-you turn over the 7 to be sure there isn't a D on the back
***************


There may be other replies also, but you will not receive any more notifications until you visit the forum again.

All the best,
Bad Astronomy and Universe Today Forum

;)

[Sarawak]

I'm with the "D" and "7" people.

[caveman1917]

If another assumption is that the rule only applies to the four given cards: D and 7.

If that assumption is false, then it cannot be solved.

ETA:

(D => 3) one card should be checked for this (D)
=> (!3 => !D) one card should be checked for this (7).

[Sarawak]

If that assumption is false, then it cannot be solved.

Not following you there. Whether it applies to other cards or not seems irrelevant.

[caveman1917]

Not following you there. Whether it applies to other cards or not seems irrelevant.

Suppose those are 4 cards drawn from a pack of 10. Then we cannot confirm the rule (as it would apply to the whole pack) using only those 4 examples.
The problem statement was a bit ambiguous as to that. At least i read it in that way first, so my first response would've been: you cannot solve this by only having some examples.
Then i realized it was meant that the we need to confirm the rule only as it applies to those 4 given cards.

[Sarawak]

Suppose those are 4 cards drawn from a pack of 10. Then we cannot confirm the rule (as it would apply to the whole pack) using only those 4 examples.

Ah. Got you.

[clop]

Suppose those are 4 cards drawn from a pack of 10. Then we cannot confirm the rule (as it would apply to the whole pack) using only those 4 examples.
The problem statement was a bit ambiguous as to that. At least i read it in that way first, so my first response would've been: you cannot solve this by only having some examples.
Then i realized it was meant that the we need to confirm the rule only as it applies to those 4 given cards.

Certainly that was the way the other 35 people managed to understand the question.

clop

[caveman1917]

Certainly that was the way the other 35 people managed to understand the question.

Apparently 33 :)


Assuming that the rule only applies to this set of four cards {...}

[Sarawak]

Certainly I have cards with "3" written on one side, but no "D" written on the other. So I would only need to turn over that card to confirm that the rule does not apply universally. But, I have used prior knowledge of what is written on that card, so I guess that is cheating.