Frieda noticed that 3/4 of cars are sedans and that half of them are white what fraction of the sedans are white

If "them" is referring to "cars", cannot be determined from the information given. If 3/4 of cars are sedans and 1/2 of cars are white, anywhere from 1/3 to 2/3 of sedans are white.

If "them" is referring to sedans, 1/2 of sedans are white.