Answer:
10.5
Explanation:
A proportion shows data in relation to each other.
12:18 as 9:__ (read 12 is to 18 as __ is to 9)
15: 22.5 as 12:18 (read 15 is to 22.5 as 12 is to 18)
With each proportion the difference is 1.5, so if you add 1.5 to 9, you get 10.5.
So,
9:10.5 as 12:18 as 15:22.5 (read 9 is to 10.5 as 12 is to 18 as 15 is to 22.5)