Final answer:
The ratio of the number of English books to the number of Math books is 9:10.
Step-by-step explanation:
To find the ratio of the number of English books to the number of Math books, we divide the number of English books by the number of Math books. In this case, there are 81 English books and 90 Math books. So the ratio is 81:90. To simplify this ratio, we divide both sides by 9 (the greatest common factor). Therefore, the simplified ratio is 9:10.