Problem with calculations for enzyme activity

[maja_l]

I know it sounds trivial but i have some problems with calculations for the experiment that I've performed today in the lab. I was supposed to do the bioassay for the lysozyme, using 3 samples: crude, precipitation, dialysis and of course standards.

 

Preparation of satndards: 1mg lysozyme/ml of stock with/ spec activity of 47080 U/mg that needed to be diluted 20 times (I've used a total volume of 500ul, so 25ul in 475 ul of potassiumphosphate buffer)

 

Stand 1:

150 ul of potassiumphosphate buffer

150ul of diluted lysozyme stock

 

then what is enzyme activity in the standard solution (U/ml)?

I keep making some mistakes and i'm pretty sure of it