@Christian Limbago, It appears that you don't have Google+, so no one can reply to your question. The easiest way to understand this is to draw a venn diagram. The reason that, B-A=B(intersection)A^c, is as follows: B-A is the subset of all elements that are ONLY in B, but NOT in A, B(intersection)A^c, are the elements that are both in B, AND/BUT NOT A. [I.E. Only those elements of B, which are also A^C. Since A^C excludes all elements in A, we are left with only those elements of B which are not in A, which would be the same as B-A.]
for the last example, can't you just subtract? P (A' U B) - P (A' • B') = P (B). (A' • B') is subset to (A' U B), and is everything outside of B (all the bad stuff we'd want to subtract to be simply left with B). Doing a Venn Diagram showed this to my eyes. Or simply, 0.8 - 0.25 = 0.55. did I just discover an interesting relationship?
