Question 391255
Let {{{a}}} = liters of acid to add 
given:
{{{.15*6 = .9}}} liters of acid in 15% solution
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In words:
(liters of acid)/(liters of solution) = 40%
{{{(a + .9) / (a + 6) = .4}}}
{{{a + .9 = .4*(a + 6)}}}
{{{a + .9 = .4a + 2.4}}}
{{{.6a = 1.5}}}
{{{a = 2.5}}}
2.5 liters of pure acid must be added
check answer:
{{{(a + .9) / (a + 6) = .4}}}
{{{(2.5 + .9) / (2.5 + 6) = .4}}}
{{{3.4/8.5 = .4}}}
{{{3.4 = 3.4}}}
OK