Question 71265
Let x=amount of the 99.5% solution to be added

Now we know that the amount of pure acetic acid in the original solution (0.1(100)) plus the amount of pure acetic acid in the solution that was added (0.995(x)) must equal the amount of pure acetic acid in the final mixture ((100+x)(0.28)).  So our equation to solve is:

0.1(100)+0.995(x)=(100+x)(0.28)  get rid of parens

10+0.995x=28+0.28x  subtract 10 and also 0.28x from both sides

10-10+0.995x-0.28x=28-10+0.28x-0.28x  collect like terms

0.715x=18  divide both sides by 0.715
x=25.175 or 25 parts of the 99.5% solution must be added----------------ans

CK

0.1(100)+0.995(25)=125(0.28)
10+24.875=35
~35=35


Hope this helps------ptaylor