My Favourite Logical Problems


The Wises Council

Once, upon a time, it was a famous wise named Hasen Said. The people named him "The Wise of the Wises". One day, while travelling in the world, he arrived at a sultan's Court, where he was received with high honour. The sultan treated him with food, lodged him and, one day, said to him:

"I govern my country with the help of the Wises Council, who is said to gather the 12 wisest men in my country. However, can you put them to a test so that they can succeed only if they are worthy of the honour I give them?"

Hasen Said thought a bit and then said:

"All right, gather them."

When all the wises have been gathered, Hasen said to them:

"Oh, Wise Men, your king, the sultan, gathered you to show us all your wisdom. The servants had put a box before everyone of you. All these boxes are identical. Here, in my bag, I have twelve precious stones: some of them are emeralds, the others are rubies. I invite everyone of you to get out of this room, while I put one of these stones in his box, so that everyone can see the others' stones, but he doesn't know his own stone."

And Hasen invited the first of the Wises to leave off the room. He put in his bag a ruby. After he returned, the second left and Hasen put in his box an emerald. To the third, he put an emerald. And so on until the last. When the last Wise returned, Hasen said to them:

"Everyone of you saw the others' stones, but he doesn't know his own stone. If you are wises indeed, and if you trust your mind and your eyes, nothing prevent you to carry out my wish: all of you who have emeralds, come here and put your box to the sultan's feet."

Unfortunately, nobody comes. The sultan got then angry and ordered that all his Wises Council be banished from the Court, but Hasen stopped him:

"Don't act rashly, Sir. Me too I would have done the same thing."

Ten minutes later, Hasen said to them again:

"All of you who have emeralds, come here and put your box at the sultan's feet!"

The same silence, nobody came. Hasen repeated the same invitation every ten minutes, and after an hour, some of the Wises raised up and came to the sultan, who opened their boxes: they all contained emeralds! He asked to see the others' boxes: there were all rubies. And we have to say that while all this hour, the Wises didn't say anything, they only thought.

"Oh, Sir, Hasen said to the sultan, you may be proud of your Wises Council. They are really wise men!" And saying them good bye, he left them.

The story ends here, but we invite you to continue it, showing how many emeralds Hasen had put into the Wises' boxes and how did they find out what stone he owns...

Romanian version:

Sfatul înteleptilor

Traia de mult un intelept pe nume Hasen Said, despre care mersese vestea in toata lumea. Oamenii il numeau Inteleptul-intre-intelepti. Si cum pornise el prin lume, ajunsese intr-o buna zi la curtea unui sultan, unde fu primit cu toata cinstea ce i se cuvenea. Sultanul il ospata, il gazdui, iar intr-una din zile ii spuse:
- Treburile tarii le conduc având lânga mine intotdeauna Sfatul inteleptilor. In Sfatul acesta am ales doisprezece oameni, despre care se spune ca ar fi cei mai luminati la minte din intreaga tara. Totusi, prea-inteleptule Hasen, n-ai putea sa pui tu Sfatul la o asemenea incercare, pe care s-o poata dezlega numai daca fiecare va dovedi o intelepciune demna de increderea pe care le-am acordat-o?
Hasen Said se gândi putin si rosti:
- Bine, aduna-ti Sfatul.
Când intregul Sfat al inteleptilor se strânse in sala tronului, Hasen li se adresa astfel:
- O, inteleptilor, stapânul nostru, Sultanul, v-a adunat aici ca sa aratati intreaga stralucire a mintii voastre prea luminate. Slugile au pus in fata voastra câte o cutie. Toate cutiile sunt la fel. In saculetul acesta am douasprezece pietre pretioase: unele sunt rubine, altele - smaralde. Eu o sa va rog ca fiecare dintre voi, pe rând, sa iasa o clipa din sala, iar in absenta lui o sa-i pun in cutie fie un rubin, fie un smarald.
Si Hasen Said il invita pe primul dintre intelepti sa paraseasca sala. Lui ii puse un rubin. Urmatorilor doi le puse câte un smarald. Astfel, iesira unul dupa altul toti inteleptii, iar pietrele cele scumpe fura puse, una cate una, in cutii. Când reveni in sala ultimul intelept, Hasen le spuse:
- Fiecare dintre voi a vazut ce fel de piatra am pus in cutia celorlalti, dar nici unul nu stie ce se afla in cutia lui. Daca voi sunteti cu adevarat intelepti, daca ochii si memoria nu va insala, nimic nu va va impiedica sa indepliniti rugamintea mea: toti cei in a caror cutii li s-a pus câte un smarald, sa ia cutiile si sa le puna la picioarele sultanului.
Asa grai Inteleptul-intre-intelepti, totusi cei carora li se adresase ramasesera nemiscati la locurile lor. Sultanul se facu rosu de mânie si dadu porunca sa fie izgoniti din palat toti cei ce faceau parte din Sfatul inteleptilor, dar Hasen il opri:
- Nu te pripi, maria ta. Si eu as fi facut intocmai ca ei.
Peste zece minute Hasen li se adresa din nou inteleptilor:
- Cel ce are in cutia lui un smarald, sa aduca cutia cu pretioasa piatra la picioarele sultanului!
Aceeasi liniste, nimeni nu facu nici macar un gest. Hasen repeta invitatia la fiecare zece minute, iar când se implini un ceas de la prima rugaminte, o parte dintre intelepti se ridicara si adusera cutiile sultanului. Acesta le deschise - inauntrul lor, numai smaralde. Ceru sa vada si cutiile celor ce ramasesera la locurile lor. Toti aveau numai rubine. Si trebuie sa mai spunem ca in tot acest ceas, nici unul dintre intelepti nu scoase o vorba, nu intreba nimic.
- O, maria ta, zise Hasen dupa ce se incheie incercarea, poti fi mândru: in Sfatul tau stau numai oameni care merita sa poarte titlul de intelept.
Si luându-si ramas bun de la toti, pleca pe drumul sau.
Povestea se termina aici. Pe dv. insa va invitam sa o continuati, aratând câte smaralde a pus Hasen Said in cutiile inteleptilor si cum au ghicit fiecare dintre acestia ce piatra se afla in fata lui?

Sursa: Dan LAZARESCU - "PALEOARITMETICA si alte probleme de logică"


12 coins

You are given 12 identical-looking coins, one of which is counterfeit and weighs slightly more or less (you don't know which!) than the others. You are given a balance scale which lets you put the same number of coins on each side and observe which side (if either) is heavier. How can you identify the counterfeit and tell whether it is heavy or light, in 3 weighings? More generally, you are given N coins, one of which is heavy or light.

Romanian version:

12 bile

Presupunem 12 bile din care una are o greutate diferită. Din 3 cântăriri cu o balantă să se descopere bila respectivă si dacă este mai grea sau mai usoară. Generalizare.

Sursa: PC Report, febr. 1997 (& Martin Gardner)