Janvisahu wrote:
Hi
GnpthStatement 1 :
probability of both event A & B = least value of A or B
this is because P(A& B) = P(A)*P(B) , if the events are independent.
P(A& B) = P(A)*P(B/A) , if the events are independent.
now since the max value of probability for any event P(B) or P(B/A)= 1, hence
either P(A) = 0.6 and P(B) = 1 , or P(B) = 0.6 and P(A) = 1, in both the cases, P(A) > 0.5
Answer should be A, please confirm
Dear
Janvisahu,
I'm happy to respond.
My friend, I'm sorry, but your reasoning is flawed. Basically, you were assuming that Statement #2 was already true as you were reasoning with Statement #1.
Let's be clear: a formula-based approach to probability is doomed to failure. All the formulas in probability are context-specific, and if you don't understand exactly when you can and when you can't use one, then are driving blind through the realm.
The formula P(A and B) = P(A)*P(B) is
ONLY true in the
very rare and special case events A & B are independent. Most things in the real world are not independent, and if A & B are not independent, then that formula is entirely out-the-window.
For two events to be independent, truly independent, means that if we know the outcome of event A, that gives us absolutely no information about the outcome of event B, and vice versa. The following are real-world event pairs that are
not independent
1) A = person earns over $200K annually, B = person has a college-degree or higher
2) A = a person eats pork, B = a person is a Muslim
3) A = a person has smoked for years, B = a person gets cancer
None of these pairs are independent, because if we know the answer to one question, that answer automatically makes the other event either more likely or less likely.
As it turns on, typically only artificial inanimate things are truly independent, such as the dots showing on two different dice rolled together, or two successful flips of a coin. A few very unusual rare things in the real world would be truly independent. For example, if A = a person is left-handed, and B = a person gets cancer--so far as I know, those two are entirely independent, because knowing the answer to one question leaves us totally in the dark about the answer to the other question.
Thus, most real events are NOT independent, and so the P(A and B) = P(A)*P(B) is useless and 100% inapplicable most of the time. We can only use that in the very special case in which A & B are guaranteed to be independent. Under statement #1, the two events
could be independent, but they don't
have to be independent.
Back to statement #1:
St-1: The probability that events A and B both occur is 0.6Case (a): P(A) = 0 and P(B) = 0.6, so P(A and B) = 0.6. This is consistent with Statement #1 gives a prompt answer of no.
Case (b): P(A) = 0.8, P(B) = 0.75, events & A & B are independent: then P(A and B) = 0.6 This is consistent with Statement #1 gives a prompt answer of yes.
Consistent with Statement #1, we can construct scenarios that produce either a yes or no answer to the prompt. Thus, statement #1, alone and by itself, is
not sufficient.
Statement #2 is
not sufficient by itself either, but when we combine the statement, then we get the reasoning that you provided. Together, the statements are
sufficient, and the correct answer is
(C).
Does all this make sense?
Mike