Find The Thief
The famous Blue Diamond was stolen from a museum. A thief swapped it for a cheap green imitation diamond. The day it was stolen, 2000 people entered the diamond room one by one. Inspector Bebro must find the thief by interviewing these people. Bebro has a list of all 2000 people in the order they entered the room. The only question Bebro is allowed to ask is was the diamond green or blue when you saw it? The thief will lie and say green but everyone else will tell the truth.
Bebro claims "I can find the thief by interviewing at most 20 people!" Is this claim true or false?