is the school answer wrong?

OP changed the order of two lines of in the reply. The 19/20 line which I was looking at, is the second line of the original thread whose conditional probability denominator is spam ("GIVEN that it is a spam email", so it's spam classified + spam non-classified) while OP claims in the reply that it's "classified spam" and "classified non spam". When this 19/20 is misunderstood, it's natural to say the other 38/45 is the same.

At least so far none of us has edited thread texts so you'll be looking at the same thing as I did.

Either you weren't paying enough attention to notice that, in which case I can totally forgive, or you are his "classmate" or even OP's alt, in which case I don't care.

/r/learnmath Thread Parent