If I understood correctly:
on 7 out of 10 days it's raining? so on 3 out of 10 days it is not raining. (10-3=7)
and the question is: in 90 days, on how many of those would it rain and on how many not? ( correct me if I'm wrong, I think You had some typos and I couldn't understand the question)
so in 90 days there are 9 times 10 days. If in every one of those 10 days on 7 it's raining, then it's raining on 9*7=63 days!
and days when it is not raining are: 90-63=27!
