how to convert "units"of reagent in weight ?

[M D]

Excuse me for this very trivial question, but I have a doubt on my method to 
calculate a concentration using international units (IU). For instance, in a 
paper from Davidson and Cronin on NAP culture media, the composition of 
polymyxin B solution is 0,03 % w/v. But this reagent is sold in vials 
containing 30,000 units per vial. What is the exact way to convert these 
units ?

Sometimes, those simple questions are ruining your day...

Thanks !

M.D.
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