Let's use the formula for calculating probabilities of events:
P(A or B) = P(A) + P(B) - P(A and B)
where A and B are two events.
Let A be the event that it is windy on Wednesday, and B be the event that it is windy on Thursday.
We know that:
P(A) = 0.15 (the probability that it will be windy Wednesday)
P(B) = 0.65 (the probability that it will be windy Thursday)
P(A and B) = 0.95 (the probability that it is windy both Wednesday and Thursday)
To find the probability that it will not be windy either day, we need to find the complement of the event that it is windy on at least one of the two days. In other words:
P(not A and not B) = 1 - P(A or B)
Now we can plug in the values we know:
P(A or B) = P(A) + P(B) - P(A and B)
P(A or B) = 0.15 + 0.65 - 0.95
P(A or B) = -0.15
Since the probability is negative, we know that we made a mistake. The mistake is that we are counting the probability of being windy on both Wednesday and Thursday twice, once in P(A) and once in P(B). We need to subtract it only once, so the correct formula is:
P(A or B) = P(A) + P(B) - P(A and B)
P(A or B) = 0.15 + 0.65 - 0.95
P(A or B) = -0.15 + 1
P(A or B) = 0.85
Now we can find the probability that it will not be windy either day:
P(not A and not B) = 1 - P(A or B)
P(not A and not B) = 1 - 0.85
P(not A and not B) = 0.15
Therefore, the probability that it will not be windy either day is 0.15, or 15%.