A 10% salt solution is to be mixed with a 20% salt solution to produce 20 gallons of a 17.5% salt solution. How many gallons of the 10% solution and how many of the 20% solution will be needed?

Amounts: x+y=20
Concentrations: 0.1x+0.2y=0.175(20) 
for Concentration we multiply by 10: x+2y=35 
next we subtract the amounts equation to get
y=15 and from there we deduce x=5, 


:)