Hi, I'm quite confused on the formulas for finding different probabilities. So i used to think that p(A n B) = p(A) x p(B), and that p(AuB)=p(A)+p(B), but then now in S1 i keep coming upon the formula p(AnB) = p(A n B) = P(A) + p(B) - p(a u b). Im really confused on which one to use, and how do you know which to use?
For example, in the question: A and B are two events and p(A)=0.5 and the P(b)=0.2 and p(A n B) = 0.1, find p(A u B). So surely the probability of a and b is 0.5 multiplied by 0.2 which is 0.1 as the question suggests, but then if you do 0.5 add 0.2 for the question which = 0.7, this answer is wrong and you're apparently meant to use another formula? How do you know when and which to use?
Many thanks