Question 259795
No, the only mixtures you can obtain range from 30% to 50%. 
30% means no 50% solution added to the mix. 
50% means no 30% solution added to the mix.
Algebraically, let A be the # of gallons of 30% solution, B be the # of gallons of 50% solution.
Let's try to make 1 gallon of 70% solution. 
{{{A+B=1}}}
{{{30A+50B=70}}}
From eq. 1, 
{{{A=1-B}}}
{{{30(1-B)+50B=70}}}
{{{30-30B+50B=70}}}
{{{20B=40}}}
{{{B=2}}}
Oops I need 2 gallons of B to make one gallon of 70% solution. 
Now if you solve for A.
{{{A=1-B=1-2=-1}}}
Wow now I need -1 gallons, how do you do that?
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It's not possible, you need to increase the concentration of the solution to add least 70% or it won't work.