Question 780824
Let C = capacity of the vessel
p = percent concentration initially filled in the vessel
T = percent concentration wanted filled in vessel after adjustment
h = percent concentation of material to add
v = volume of the p concentration material to drain and replace with the h material; unknown


{pure matter}/{total mixture}={concentration}


{{{highlight((Cp-vp+vh)/C=T)}}}
Simplify and solve for v.
{{{Cp+vh-vp=CT}}}
{{{v(h-p)=CT-Cp}}}
{{{highlight(v=C(T-p)/(h-p))}}}


Substitute according to 
h=100 %
p=25 %
T=60 %
C=6 Liters
and then compute v for the exercise.