Answer: 2150 books
Step-by-step explanation: Let the number of books Jessica has be the variable x. Percentage works the same way as multiplying fractions, for example 19% of "something" is 19/100 times that "something". If Jessica has x books, and Lily has 15% more books (that is, 115% Jessica's books), at the same time Lily has 150 more books, it's safe to assume that 15% of x is 150, and so, 15x/100 = 150, if so, 15x = 15000, and x = 1000. Now that Jessica has 1000 books, and Lily has Jessica's books and 150 more (or 1000 + 150), the problem sums up to (1000 + 150 + 1000) = 2150.