When Is Cheryl's Birthday?

Iannis: I think you need to reread The Ancient's solution carefully and realize that through clever use of post-measure grouping the outlier is identified even if we don't know at first whether he is lighter or heavier.

WAIT... wait... 3a) is messed up I think. Let me reread it carefully.

Yeah 3a) is fubared because a single weighting is given 4 different outcomes instead of 3.

There is an extra D on both sides in 3a. You can disregard it.

Yeah you are right, 3a does fail. Oh well.

If you really want to know the answer just google twelve coins problem. Pretty old logic puzzle.

I checked the wikipedia article to be sure we have the right formulation for the problem and we do. My ego is hurt a little by the fact the following paragraph started with "There is more than one solution to this problem..."

Szlia_sl said:

I checked the wikipedia article to be sure we have the right formulation for the problem and we do. My ego is hurt a little by the fact the following paragraph started with "There is more than one solution to this problem..."

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I face my personal dumbening every day. It no longer bothers me. Bring on the dementia.

I get 3 v3 needing 4 steps to solve if equal initially ...3 Equal, move to one side,6 v 6 the heavy or light, 3 knock in half, 1+1+1


5 v5 if equal, take one off each side add one if equal last guy is odd weight if heavy or light it is guy who just got on
5 v5 unequal, takes up to like 5 or 6 steps
4 v4 equal 3 steps -4 v equal, 4 v for h/l(using 5 from first group 3 remainder) if equal it is last guy, if not then 1+1+1 for step 3
4 v4 unequal, 4 steps - 4 v4, take off light group, 4 v other4 for h/l(if equal 4 took off is the l group or group still on the h), 2 v2, 1 v1

Wait a tic, this Cheryl's birthday puzzle doesn't make sense.

I get that Albert can cross off May 19 and June 18 because Bernard was told the day, and since 18 and 19 are exclusive, if he was told 18 or 19 he would know right away. Since he didn't know right away, you get rid of May 19 and June 18.

I get that it can't be June 17, because Albert can determine that Bernard wasn't told 18 or 19, and if Albert had been told June, June 17 would be the only day left.

But I don't understand how you guys are writing off May 15 and May 16th. I don't see how May 19 not being a possibility also stops May 15 and 16 from being possibilities.

Like, here's how I'm following this if I translate what they're saying into a real conversation (assuming they're not allowed to tell each other the month or day they heard):

Bernard: "I have no idea when her birthday is, but I know it's not May 19th or June 18th, because I wasn't told 19 or 18 and those numbers are exclusive to those days."

Albert: "Well I wasn't told June, so I know it can't be June 17th, because after you eliminated June 18th, June 17th would be the only day left."

Where does the part about May come in?

I think what confuses people (and confused me for the last step), is that you are not trying to be in Albert's or Bernard's shoes, trying to solve Cheryl's riddle, you are an outside observer trying to find the only date for which this specific conversation between Albert and Bernard is possible.

- Albert knows Bernard does not know -> he was not told May nor June.

- Bernard knows the birthday if the month is not May nor June -> he was not told 14.

- Albert knows the birthday knowing the month and that Bernard was not told 14 -> the birthday is July 16.

Szlia_sl said:

I think what confuses people (and confused me for the last step), is that you are not trying to be in Albert's or Bernard's shoes, trying to solve Cheryl's riddle, you are an outside observer trying to find the only date for which this specific conversation between Albert and Bernard is possible.

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Good point.

Szlia_sl said:

- Bernard knows the birthday if the month is not May nor June -> he was not told 14.

Click to expand...

Okay, I think this is what I was missing. Thanks!