Write and solve an equation to answer the question below.  

Lorena and Lena both work in the library. Lorena has already returned $38$ books to the shelves and can shelve $16$ books an hour on average. Lena, on the other hand, has already returned $52$ books to the shelves and can shelve $20$ books per hour on average. If together they started with a total of $252$ books to return on the shelves, how much longer should it take them?