A grandfather has an x-linked disease. His wife does not have the disease and she is not a carrier for it. What is the probability that their daughter's son will have the disease?

There is 50% probability that their daughter's son will have the disease, because daughter chromosomes are XX, and the X from father will have x - linked disease, now in next generation the son will have XY chromosomes, Y is from father and X will from mother, now mother have two "XX", out of these two one carry x - linked disease, that means 50 percent chances of having disease in son.