Harvard-MIT Math Tournament #0hzc
From module Geometric Probability
Understand the scale
Problem
Problem:
As part of his effort to take over the world, Edward starts producing his own currency. As part of an effort to stop Edward, Alex works in the mint and produces 1 counterfeit coin for every 99 real ones. Alex isn't very good at this, so none of the counterfeit coins are the right weight. Since the mint is not perfect, each coin is weighed before leaving. If the coin is not the right weight, then it is sent to a lab for testing. The scale is accurate of the time, of all the coins minted are sent to the lab, and the lab's test is accurate of the time. If the lab says a coin is counterfeit, what is the probability that it really is?