Answer: 2 floors
Step-by-step explanation:
If there are 9 apartments on 1 floor, there are 18 apartments, which means that there are two times nine. The ratio would be 9 apartments to one floor, or 9:1. 18 is 9 multiplied by 2, so you would also have to multiply 1 by 2, giving you the answer of 2 floors.
