So, two mutually exclusive events A and B
P(A or B)=P(A)+P(B).
But what if the probabilities add up to be greater than 1? We can't subtract P(AandB).
Can P(A or B) with mutually exclusive events be greater than 1?
The point is the bit above is impossible. It cannot happen.
Consider, $\displaystyle P(A \cup B) = P(A) + P(B) - P(A \cap B) \leqslant 1$.
From the given $\displaystyle P(A \cap B) \geqslant 0.8$.
Because $\displaystyle P(A \cap B) \ne 0$ the events are not mutually exclusive.