What are the 40 hours requirements for a PPL?
There is no requirement for the remaining 10 hours, just that those 10 hours are part of your instruction/training. 

So really you can read it as "a private pilot applicant must log at least 40 hours, of which 20 must be flight training from an authorized instructor and 10 hours must be solo". 

How you distribute the remaining 10 are up to you and your instructor, although the vast majority of students don't find it difficult to place those hours anyway since the national average is closer to 70 hours to get a PPL.