Question 751344
All as weight per weight. 


w = amount of water in grams
s = amount of salt in grams


{{{s/(s+w)=0.20}}} and {{{s+w=100}}}
Two variables unknown and two equations.  Solve the system.  Note, you do not really need the second equation. You essentially have just this:


{{{highlight(s/100=0.20)}}}, and the rest of the mass to make up to 100 grams would be the needed water.  You may still use the second equation, mass balance.