How to find the conditional probability.
Muscular dystrophy is an irredeemable muscle-wasting disease. The most common as well as serious type called DMD is caused by a sex-linked recessive mutation. Exactly women can be carriers but don't get the disease; a son of a carrier has probability 0.5 of having DMD a daughter has probability 0.5 of being a carrier. As many as 1/3 of DMD cases but are due to spontaneous mutations in sons of mothers who are not carriers. Toni has one son who has DMD.
In the non-appearance of other information the probability is 1/3 that the son is victim of a spontaneous mutation and 2/3 that Toni is a carrier. There is a screening test entitled the CK test that is positive with probability 0.7 if a woman is a carrier as well as with probability 0.1 if she is not. Toni's CK test is positive. What is the possibility that she is a carrier?