Question 778390
Let v = volume to drain and replace.

{{{(15*40-v*40+v*100)/15=50}}}
{{{(15*40+60v)/15=50}}}
{{{15*40+60v=15*50}}}
{{{15*4+6v=15*5}}}
{{{6v=15(5-4)}}}
{{{6v=15}}}
{{{2v=5}}}
{{{v=2&1/2}}}