Short Answer:
Ruby's rate if reading the books is:
2 months per book.
Long Answer:
If you look at it this way, two months per book means that by the time 14 months come you should have 7 books read.
Easy check to verify:
7 (books) X( mutltiply) 2(# of books) = 14 (total of months) - This shows that the number lead back to the original numbers
Hope this helps :)