We must find the probability that a fair coin was chosen given that a head appeared using Bayes' theorem. The total probability of getting a head is the sum of the probability of choosing a fair coin and getting a head, which is (n/(n+1)) times (1/2), and the probability of choosing the biased coin and getting a head, which is 1/(n+1). This sum simplifies to (n+2) divided by (2n+2). Dividing the probability of a fair coin yielding a head, n/(n+1) times 1/2, by this total probability gives n/(n+2), which proves the assertion is true. Since Bayes' theorem is the fundamental rule for calculating such reversed conditional probabilities, it is the correct explanation for the assertion.