There are four cards. Each has a number printed on one side and a letter on the other.
There is a rule - which may or may not be true - that every card printed with 'D' on one side must have '3' printed on the other side.
The cards are laid down. They show D, F, 3, 7.
Which card(s) do you have to turn over in order to verify whether or not the rule holds good?
The solution - and the reasoning behind it - is given at around the 3-minute point in the video linked to below.
The answer may surprise you. It certainly suprised me.
Comments
my answer
turn d if not 3 rule false, if 3 turn 7 if not d rule true.
Logically Unimpeachable
"Because as soon as I was boss/The first one in line took my head clean off" - Todd Rundgren
In other words never underestimate the BC readership. Happy New Year.
Check...
Having seen the puzzle a couple times helps. Check D for 3, and check 7 (all numbers but 3) for D. (It doesn't matter what's behind 3.)
People seem to have an easier time if the Rule is, whoever drinks alcohol must be at least 21. You check alcohol-drinkers, and you check under 21s. Here's one:
-- Daphne Xu
Survey says:
Walking on one leg, you "hop." May not get very far as it is rather tiring.
The opposite of bottom is "surface." I blame this answer on my time at sea.
At the green light, you "continue on your merry way." Obviously, you only have to stop if the light is red.
_Bev_
Hmmmm
Walking on one leg, you fall. (Hopping is not walking)
The opposite of bottom is top
At the green light you go
Commentator
Visit my Caption Blog: Dawn's Girly Site
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You cannot prove the rule only disprove it
The rule which may or may not be true only says that every card with D must have a three so if you turn the D over and it is not a three you can disprove the rule but if it is a three turning any of the other cards over will not prove the rule though if the seven has a D it can also disprove the rule. It does not say that every card with a three must have a D on the other side so the card with the F can have a three and the card with the three may have any letter and the rule still applies so turning either of them over will neither prove or disprove the rule.
Commentator
Visit my Caption Blog: Dawn's Girly Site
Visit my Amazon Page: D R Jehs
Yeah
More or less
Not Entirely Sober
Didn't mean to be rude. Drinking Tokaji and grooving to Swing Out Sister. I mean, who wouldn't?
https://youtu.be/kKwyAzTvQ1I?list=PLclgAhnK-vk0-mx1JFLmWC_U6...
Happy New Year
Is the rule obeyed or not? I
Is the rule obeyed or not? I think that this is the idea here. The rule is not so much proven, as obeyed in this situation.
-- Daphne Xu
I Like It
Simple logic tests of the sort that we had to do way back in the sixties and seventies to get jobs as programmers!
So first, flip 'D' to see if the rule holds true.
Forget 'F' since the rule does not concern itself with that letter.
And, because of the wording which does NOT state that every '3' must have a 'D', then the '3' can be ignored too.
The only other card needing testing is the '7'. If that has a 'D' on the other side, then the rule does NOT hold.
So two cards should be flipped - the 'D' and the '7'.
Fun for the end of the year, thanks.
J