lets take
X= Number of books that sylvia sold = 100%
Now, Sam's sold 15% more books than sylvia which makes it that sam's sold (100%+15%) =115%
when taking % off( simply you devide by 100-----> 115/100 =1.15) that amount of books sold by Sam's =1.15X
1.15x=69 books
X=60= number of books sold by Sylvia.