Answer:
7:9
Explanation:
The basic ratio is 63:81 but to simplify this ratio you can simplify each number as a multiplication of its factors.
63=3*3*7
81=3*3*3*3
Now find the factors they have in common and get rid of them.
63=3*3*7
81=3*3*3*3
The numbers in bold are all the factors that they have in common.
You can cancel these numbers out.
Now the remaining numbers you can put into a ratio in the same order
7:3*3
7:9