Answer:
For every 7 books Ethan read, Sadie read 3 books.
Step-by-step explanation:
For every 21 books Ethan read, Sadie read 9 books.
Unsimplified Ratio = 21:9
21 and 9 have a common factor of 3.
Because 21 and 9 share a common factor we can simplify the ratio
21 / 3 = 7
9 / 3 = 3
The new ratio is 7 to 3
So for every 7 books Ethan read, Sadie read 3 books.
