Question 315380
Let {{{x}}} = ml of acid to be added
In words:
(final ml of acid)/(total ml of solution) = 70%
{{{(300 + x)/(600 + x) = .7}}}
{{{300 + x = .7*(600 + x)}}}
{{{300 + x = 420 + .7x}}}
{{{x - .7x = 420 - 300}}}
{{{.3x = 120}}}
{{{x = 400}}}
She must add 400 ml of acid