The probability that event A occurs is 0.4, and the probability that events A and B both occur is 0.25. If the probability that either event A or event B occurs is 0.8, what is the probability that event B will occur? Choose the correct choice from the following answers.
A) 0.05
B) 0.15
C) 0.45
D) 0.5
E) 0.65

Rationale: "p ( a or b ) = p ( a ) + p ( b ) - p ( a n b ) 0.8 = 0.4 + p ( b ) - 0.25 p ( b ) = 0.65 ans : e"
Answer: E