Question 664932
let x be the number of liters of 20% solution to be added. Then
{{{0.2x + 0.7(20) = 0.6(20 + x)}}}
{{{.2x + 14 = 12 + 0.6x}}}
{{{2 = .4x}}}
{{{5 = x}}}

you need to add 5 liters