Hi Gmat_mission.

Let's take a look at your question.

Let's call "x" the rainfall in the first week.

"the rainfall during the second week was 1.5 times the rainfall during the first week" implies that the rainfall during the rainfall during the second week was 1.5*x.

"During the first two weeks of January, the total rainfall in Springdale was 15 inches" this implies the following: $$x+1.5\cdot x=15\ \ \ \Rightarrow\ \ \ 2.5x=15\ \ \ \Rightarrow\ \ \ x\ =6.$$ Therefore, the rainfall during the first week was 6 inches and this implies that the rainfall during the second week was 1.5*6=9 inches.

Hence, the correct answer is C .

I hope this explanation may help you.