Rules
Terms of Use

Topic Options
#1070901 - Tue Oct 28 2014 04:11 AM Alice 2
baldhair Offline
Explorer

Registered: Tue Aug 05 2014
Posts: 81
Loc: Gloucestershire UK
It was my sister Alice's birthday, and I had a present for her. As you know, Alice is a very intelligent girl, and as you also know I like presenting little problems. So I gave Alice three boxes, one wrapped in red, one in yellow and one in blue. In one box, I told her, is your birthday present; the other two are empty. "Now," I said, "you have to determine in which box is the present. You can ask me any number of questions answerable by yes or no, and I'll either answer them all truthfully, or else I'll lie each time." First, she asked if the present was in the yellow box. I answered her, yes or no. Then she asked if it was in the red box, and again I answered yes or no. And Alice, after some thought, knew which box contained the present. How?

Top
#1070982 - Tue Oct 28 2014 04:24 PM Re: Alice 2
deputygary Offline
Enthusiast

Registered: Thu May 24 2007
Posts: 283
Loc: South Dakota USA
So when she asked you if it was in the yellow box, you didn't say "Yes or no". You either said "Yes" or you said "No" but you're not telling us what your answer was. Is that right?
_________________________
"When you argue for your limitations, they become yours."--Richard Bach, Illusions

Top
#1071015 - Wed Oct 29 2014 02:35 AM Re: Alice 2
baldhair Offline
Explorer

Registered: Tue Aug 05 2014
Posts: 81
Loc: Gloucestershire UK
Yes that's right. My answer was either yes or no in each case, but you don't know which. Alice, of course, does.

Top
#1071019 - Wed Oct 29 2014 04:10 AM Re: Alice 2
Gil_Galad Offline
Forum Adept

Registered: Wed Nov 16 2005
Posts: 102
Loc: Greece
The only way she would know which box contained the present is if you gave the same answer in both questions. "Yes, Yes" means you were lying and "No, No" means you were telling the truth, and in either case the present would be in the blue box. If you answered one "Yes" and one "No" (regardless of the order) it would be in one of the first two boxes, but Alice wouldn't know which one, since she doesn't know if you're lying or telling the truth.

Top
#1071099 - Wed Oct 29 2014 01:59 PM Re: Alice 2
baldhair Offline
Explorer

Registered: Tue Aug 05 2014
Posts: 81
Loc: Gloucestershire UK
Spot on, Gil Galad. Well-reasoned.

Top

Moderator:  ozzz2002