Question 753448
I edited your question for spelling and identity of units of volume.  


let v = volume of water to add.


{{{highlight((300*50+v*100)/(300+v)=90)}}}
{{{15000+100v=90(v+300)}}}
{{{15000+100v=90v+27000}}}
{{{10v=27000-15000}}}
{{{v=2700-1500=1200}}}
{{{highlight(v=1200)}}} milliliters