The total amount of different books is 16.
Take away 1 from each, representing the same book they all read.
7 - 1 = 6 , 6 - 1 = 5 , 5 - 1 = 4
Add the three together.
6 + 5 + 4 = 15
Add that one book everyone read.
15 + 1 = 16 different books.
Hope I helped you out.