effisk
thou shall read the book
- Joined
- Oct 24, 2005
- Messages
- 391
okay.Slraep said:You have 80 pearls. One is lighter than all the others. Can you find the odd pearl in just four weightings? Explain why or why not.
Divide your pearls into 2 groups of 27 pearls and 1 group of 26 pearls. Weight the two groups of 27 pearls (First weighting). If one is lighter than the other, the light pearl is in that group, if not, the light pearl is in the third group (containing 26 pearls).
Then divide the group containing the light pearl in three equal parts (each containing 9 pearls - add a pearl from another group if that group only contained 26 pearls). Weight two parts (Second weighting). If one is lighter than the other, the light pearl is in that group, if not, the pearl is in the third group.
Divide the group containing the light pearl in three equal parts (3 pearls each) and do the same as above. (third weighting).
you end up with three pearls. Weight two pearls (fourth weighting), if they are of equal weight, the third pearl is the light one.