Hey there!
500 mg of protein is present in 100 mL of solvent as per the concentration 0.5 mg/mL or 500 g/mL ,
So, 250 mg (0.25 g) of serving food need to be added to 100 mL solvent in order to prepare 50 mg of protein/100 mL solution.
Dilution factor = initial amount of protein / final amount of protein
= 6 g / 0.05 g = 120
Hope this helps!