Question

A computer virus is trying to corrupt two files. The first file will be corrupted with probability 0.4. Independently of it, the second file will be corrupted with probability 0.3. Compute the PMF of X, the number of corrupted files.
A = probability of corrupted first file
B = probability of corrupted second file

Answer 1

for one file: For X = 1, 

P(~A and B)

or 

P(A and ~B) = 

0.6 * 0.3 + 0.4*0.7 = 0.46

Oh, I think I see, the universe for P(A and B) is first 

or 

second file while P(~A and B) or P(A and ~B) the universe is first, second or no file.