Thank you for the reply!
So the system1 is RNRNNNNNRR therefore we have 4 relevant documents. P@4 = 4/10?
Or for some reason we just consider the top 5 documents? In that case i would understand the 1/2...
You just consider the first k documents.
k = number of relevant documents
You look at the first k=4 documents: RNRN
So R-precision(system1) = 2/4 = 1/2