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## What is the probability that events A and B both occur?

This topic has 5 member replies
sanjib Master | Next Rank: 500 Posts
Joined
10 May 2009
Posted:
109 messages

#### What is the probability that events A and B both occur?

Fri Sep 04, 2009 8:11 am
What is the probability that events A and B both occur?
(1) The probability that event A occurs is 0.8.
(2) The probability that event B occurs is 0.6.
I think the answer is 1-(both do not occur) so in order to get both do not occur we need both st.1 and st.2.Therefore it should be C.
But its not the OA. Am I missing something.

Nermal Senior | Next Rank: 100 Posts
Joined
21 Jun 2009
Posted:
77 messages
7
Sun Sep 06, 2009 1:44 am
You can only calculate the probability if you know that the occurence of one event is independent from the occurence of the other event.

Then you could use formula P(A and B) = P(A)*P(B), it would be 0,8*0,6=0,48 (C)

But since this fact is not explicitly stated we cannot assume any indepence of the occurence of those events, therefore we don't know:

qwe12 Senior | Next Rank: 100 Posts
Joined
30 May 2009
Posted:
60 messages
2
Sun Sep 06, 2009 1:27 am
sanjib wrote:
What is the probability that events A and B both occur?
(1) The probability that event A occurs is 0.8.
(2) The probability that event B occurs is 0.6.
there are 3 formulas joint probability
http://en.wikipedia.org/wiki/Probability

p(a or b) = p(a) + p(b) - p(a and b)

we do not get p(a and b) from (1), (2) or (1) and (2)

aakaps Junior | Next Rank: 30 Posts
Joined
07 Jul 2009
Posted:
27 messages
1
Target GMAT Score:
780
GMAT Score:
740
Sat Sep 05, 2009 6:53 pm
good question!! I just came back from my usual starbucks joint where I solve my practise problems and faced exactly same question.

So the answer is.....1-both dont occur means....both will NOT occur, together or seperate. Whereas the question is both to occur together.

So the answer has to be probability of first to occur and then probability of second to occur. Both will multiply. Also you need to check whether the sequence matters. If yes, then multiply by 2, else you have your answer.

bharathh Master | Next Rank: 500 Posts
Joined
27 Aug 2009
Posted:
159 messages
19
Sat Sep 05, 2009 6:58 pm
I would have gone for C as well.

I'm guessing it's E because you don't know if A and B are mutually exclusive events... ie. If A occurs B will not occur and vice versa.

If they are not mutually exclusive, it is not possible to calculate probability.

sanjib Master | Next Rank: 500 Posts
Joined
10 May 2009
Posted:
109 messages
Sat Sep 05, 2009 10:24 pm
Please need more explanation.Because the OA is E.

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