Question 323379
Let A be the concentration of the first container, B the second container.
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{{{300A+600B=900(15)=13500}}}
1.{{{3A+6B=135}}}
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{{{100A+500B=600(12.5)=7500}}}
2.{{{A+5B=75}}}
Multiply eq. 2 by (-3) and add to eq. 1 to eliminate A.
{{{3A+6B-3A-15B=135-225}}}
{{{-9B=-90}}}
{{{highlight(B=10)}}}
Then from eq. 2,
{{{A+50=75}}}
{{{highlight(A=25)}}}
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The first container is a 25% solution and the second container is a 10% solution.