<PRE><B>Question 815449</B>
I will answer this as weight per weight, and NOT as weight per volume.  


15 ml of water can be taken as 15 grams of water, assuming density near enough to 1.00 gram per milliliter.


Let m = mass as grams of powder to add to get 50% solution, 
{{{m/(m+15)=0.50}}}
{{{m=0.5(m+15)}}}
{{{m=0.5m+15/2}}}
{{{m-0.5m=7.5}}}
{{{0.5m=7.5}}}
{{{highlight(m=15)}}}
</PRE>