A batrachologist is studying genetic mutations in a population of tree frogs, specifically three common mutations denoted A, B, and C. For each of the three mutations, the probability is .2 that a frog in the population has only that mutation (and not the other two). For any two of the three mutations, the probability is .05 that a frog has exactly these two mutations (but not the other one). The probability that a frog has all three genetic mutations, given that it has mutations A and B, is 2/3. What is the probability that a frog has none of the three genetic mutations, given that it does not have mutation C?