Chelsea adds a 65% acid solution to 25 milliliters of a solution that is 30% acid. How much of the 65% acid solution should be added to create a solution that is 40% acid?

10 ml

Explanation:

The concentration of the acid solution Chelsea adds to the 25 milliliters of 30% acid solution = 65%

The concentration of the acid solution Chelsea requires = 40%

The volume, X, of the 65% acid solution required is given as follows;

65% × X + 30% × 25 = 40% × (25 + X)

As fractions, we have;

0.65 × X + 0.3 × 25 = 0.4 × (25 + X)

0.65·X + 7.5 = 10 + 0.4·X

0.65·X  - 0.4·X = 10 - 7.5

0.25·X = 2.5

X = 2.5/0.25 = 10

X = 10 ml

The volume of the 65% acid that should be added to create a solution that is 40% acid = 10 ml.