I can follow the calculation of diseasitis - that's standard math that I learned in school. What I have a problem to follow is how you get to the "absolute propability" of 3 / (3 + 4). I think the "3+4" are the 3 parts red water and 4 parts blue water, but where does the other 3 come from? Wait … is that again the 3 parts red? So 3 Parts of 7 parts in all? Hm … I think I have solved my question ;-)