If X is N(75,100), compute the conditional probability P(PX>85|X>80)

I use the theorem sigma((b-mean)/standard deviation)-sigma((a-mean)/standard deviation) and a normal distribution probability table to get these probabilities:

.8413 for X > 85

and

.6915 for X > 80.

I see the probability of an event A given an event B has occurred is

P(A|B)=P(A union B)/P(B)

how could i find P(A union B) so i can apply this formula? Thanks!