Seattle Washington received rain on 7/10 of the day and a recent months if this pattern continues about how many days would it not rain in 90 days

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!