Visa Type and Double Entry

I am German citizen who will work in an internship in Beijing for 3 months. During my internship, I would like to visit Hong Kong for a few days, and after that, I would go back to work in Beijing. As I have checked on web, the duration of each entry is only 30 days. If I pick 'F with double entry', it seems it doesn't fit my plan as I will only be able to stay in Beijing for 30+30 days after 2 entries? I am a little bit confused.....Please guide me which is the appropriate type, many thanks! :)