Probability and Addition help On Wednesday the weather forecast for Thursday was 60% chance of rain and a 35% chance of rain on Friday. If there is a 20% chance of rain for both Thursday and Friday, what is the probability that there will be rain on Thursday or Friday? I don't know how to figure this problem out.

Let $T$ be the event of rain on Thursday, and $F$ be the event of rain on Friday.

In general, we have the inclusion-exclusion law:

$$P(A \cup B ) = P(A) + P(B) - P(A \cap B)$$

So, applied to this case:

$$P(T \cup F) = P(T) + P(F) - P(T \cap F) = 0.6 + 0.35 - 0.2 = 0.75$$