You have 9 golden acorns that look identical, but 1 of them is slightly heavier than the other 8 (which all weigh the exact same amount). Using a balance scale with two pans, what is the minimum number of weighings guaranteed to locate the heavy acorn?
2 weighings
Divide the 9 acorns into three groups of 3. Weigh Group A against Group B. If one pan drops, the heavy acorn is in that group; if they balance, it is in Group C. Take the 3 acorns from the heavy group and weigh 1 acorn against another 1. If one drops, that is the heavy acorn; if they balance, the remaining unweighed acorn is the heavy one.
About this riddle
Logic riddles give you a few facts and ask what must be true. List what you know, cross out what cannot be, and see what is left.
Difficulty: Hard. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
More logic riddles
A row goes: wave, wave, swimmer, wave, wave, swimmer, wave, wave. What comes next?
Five friends line up for a ride at the fair, and the sign lists only these facts. Felix stands ahead of Mara. Gwen stands ahead of Nico. Mara stands ahead of Gwen. Nico stands ahead of Kira. Who stands first in line?
Two mothers and two daughters went shopping together. They bought exactly three identical umbrellas, and each person walked home with her own umbrella. How is this possible without anyone sharing?
Three runners—Alex, Bea, and Carl—complete a race with no ties. Alex did not finish third. Bea finished before Carl, but Bea did not win. Who won the race?
Three boxes (Red, Blue, Green) arranged left-to-right contain a book, a coin, and a key (one item per box). 1. The key is in the Green box. 2. The coin is in a box immediately to the left of the book. Which item is in the Red box?
Five runners finish a school marathon, and only these facts survive. Jonas finished ahead of Carmen. Pavel finished ahead of Jonas. Felix finished ahead of Pavel. Carmen finished ahead of Luca. Who finished last?