<PRE><B>Question 316452</B>
Let A be the amount of 25% solution, B the amount of 35% solution.
Total quantity equation
1.{{{A+B=20}}}
Concentration equation
{{{25A+35B=32(20)}}}
2.{{{25A+35B=640}}}
Multiply eq. 1 by -25 and add to eq. 2,
{{{-25A-25B+25A+35B=-500+640}}}
{{{10B=140}}}
{{{B=14}}}
Then from eq. 1,
{{{A+14=20}}}
{{{A=6}}}
.
.
.
6 gallons of 25% solution and 14 gallons of 35% solution. 
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