A couple who are both carriers of the gene for cystic fibrosis have two children who have cystic fibrosis What is the probability that their next child will be phenotypically normal?

If both parents are carriers on cystic fibrosis, just because two children may have the disease it doesn't mean that say,a further 3 children in future will be "normal". Inheriting genes is like a lottery. I can say though that because cystic fibrosis is recessive, every offspring born under carriers will always have a 75% chance of being phenotypically normal.