Hemophilia is a -linked trait caused by a recessive allele. (“N” normal “n” recessive) A woman who is a car?

There is 50% of chance to have a male child (X from mother and Y from father), and only males could be sick (so 0% of possibility to have a female sick). There is 50% of chances that the mother give the recessive allele to his son (as there is 1 normal and 1 recessive allele). Thus the possibilities to have a son sick are 50 % of 50%, that is 25%.