Suspect

Acting on an anonymous phone call, the police raid a house to arrest a suspected murderer. They don't know what he looks like but they know his name is John and that he is inside the house. The police bust in on a carpenter, a lorry driver, a mechanic and a fireman all playing poker. Without hesitation or communication of any kind, they immediately arrest the fireman. How do they know they've got their man?

Coin Box

Given N boxes each box carry M number of gold coins. And each gold coin weights 2 grams. Among N boxes 1 box is miss placed with box carrying gold
coins of weight 1 gram.

Find the solution to find the miss placed box among N boxes.