First you add the fraction of the book she read on Friday and Saturday.
1/5 + 2/7 = 7/35 + 10/35 = 17/35
Okay this means that on Friday and Saturday, she finished 17/35 of the book.
The next part says that she read 5/9 of the pages that remain. So first we need to find how much is the remaining book by subtracting the number we found above from 1.
1 - 17/25 = 35/35 - 17/35 = 18/35
This shows us that she still has 18/35 of the book to go.
Now, in order to find 5/9 of this, we just multiply it by 5/9.
18/35 * 5/9 = 2/7
Now this means on Sunday, she read 2/7 of the book.