Set up the following equations:


x represents the amount of blue books needed, and y represents the amount of red books needed.
Solve the first equation by dividing both sides by 9:


Rebecca would need 61 pages if she used the blue book.
Solve the second equation by dividing both sides by 7:


Rebecca would need 78 pages if she used the red book.
Subtract the red book's pages from the blue book's pages:

Rebecca would need
17 more pages if she used the red book instead of the blue book.