Answer:
First, we need to find how many books there are in each group;
If we look at group 1, they have 18 books.
If we look at group 4, they have 9 books. (We got this because they have 6 whole books, and they have half of a those 6 books which is 3 books in the other book symbol, so combined: 6 + 3 = 9)
18(group 1) - 9(group 4) = 9, group 1 read 9 more books.
