Question 486071
One gallon container is full of a 75% alcohol solution. How much must be drained off and replaced by a 50% alcohol solution to produce one gallon of 65% alcohol solution?
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let x=gal of 75% solution to be drained and replaced by a 50% solution.
1-x= gal of 75% solution remaining.
..
75%(1-x)+50%(x)=65%(1)
.75-.75x+.5x=.65
-.25x=-.10
x=0.4 gal
ans:
0.4 gal of 75% solution must be drained and replaced by a 50% solution  to produce one gallon of 65% alcohol solution