1. The problem statement, all variables and given/known data
This question is problem 1.3.1 from Herbert Callen's textbook in Thermodynamics, second edition.

In the answers I have the following: Mole fraction of NaCl = 0.057, molar volume =$18\times 10^{-6} m^3/mole$.

How did they arrive at these answers? did they multiplied the ratios with Avogadro's number?

2. Relevant equations
Relative mole fraction is given by: [tex]N_k/\sum_{j=1}^r N_j[/tex] if we have a mixture of $r$ substances, where this is defined as the molar fraction;
Molar volume is defined as [tex]V/\sum_{k=1}^r N_k[/tex].

3. The attempt at a solution
How do extract the moles of each substance here?

If you are attacking thermodynamics you should already know how to convert between masses and numbers of moles, this is GenChem101 which is prerequisite.