In order for the child to be born with cystic fibrosis there needs to be two recessive alleles for the disease to develop. Based on the situation it seems that Charles is a carrier, but since Elaine's parents had one kid with and one kid without cystic fibrosis then they are carriers, meaning that Elaine can either have two healthy alleles or be a carrier as well. Therefore the probability that Charles and Elaine will give to the child the recessive allele together is that of 1/6 (1/2 from Charles x 1/3 from Elaine), making that approximately 16.7%.
