This question is about ratios. We are told that 2" of snow is equivalent to 1 " of rain and we want to figure out how much rain would be equivlaent to 9" of snow.
We can restate this as 2" snow is to 1" rain as 9" snow is to x" rain which can be writtten mathematically as
2 9
____ = ____
1 x
To solve we cross multiply and we get 2x = 9
x = 9 / 2
= 4 1/2