Murderer, Detective, and the Witness
Written on March 12th, 2025 by Sameer Wagh
20 people are caught as suspects for a murder. The detective finds out that not only do they have the murderer among the crowd, they also have a witness. The detective can bring in one or more of the suspects into a private room and ask them if they know the murderer. If the witness is in, then they will call out the murderer unless the murderer is also called in with the witness (in which case they will remain silent). What is the fewest number of interrogations that the detective has to make to find the murderer? Attempt in order: 20 calls? 10 calls? 9 calls? 6 calls? 5 calls?
Feel free to share!