Question 712055
All volumes will be as pints.  The information given also suggests concentrations are %volume/volume.  My solution (answer) assumes that.  The water contains 0% alcohol.  


P= 65%
T= 50%
V= 4 pints
v = unknown volume to add


Equation:
{{{V*P/(v+V)=T}}}
{{{V*P=(Tv+TV)}}}
{{{V*P=Tv+TV}}}
{{{Tv=VP-TV}}}
{{{highlight(v=V(P-T)/T)}}}


Substitute the values:
{{{v=(4*65-50*4)/50}}}=60/50
{{{v=1&1/5}}} pints