The actual mass fraction of an element in the mixture = the actual mass of an element/the mass of the mixture.
Theoretical mass fraction (an element in a pure substance) = actual mass of an element/mass of a pure substance.
Mass fraction of substance "mixture" = actual mass fraction/theoretical mass fraction of an element in the mixture.
= (actual mass of element/mass of mixture)/(actual mass of element/mass of pure substance)
= (actual mass of element/mass of mixture) * (mass of pure substance/actual mass of element)
= pure substance mass/mixture mass