Here, you just want to solve it by finding a common denominator.
1/5+2/7, because she read 1/5 and 2/7 of the book on separate days. The common denominator is 35.
7/35+10/35
=17/35
So that's how much of the book she read before Saturday.
Now it says that on Sunday she read 5/9 of the remaining pages, meaning:
1-17/35 should give us our answer
35/35-17/35=18/35
But since it says she read 5/9 of the remaining pages, we need to multiply 18/35 by 5/9 to get what 5/9ths of 18/35 is.
(18/35)(5/9)=2/7
Therefore she read 2/7ths of the book on Sunday.