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Re: : What is the probability that event E or event F or both w [#permalink]

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18 Mar 2011, 07:05

bogos wrote:

If we agree that 1. (E OR F) means E, or F, or both will occur 2. (E AND F) means both will occur

then we have (see MGMAT Guide 4 page 87)

P(E OR F) = P(E) + P(F) - P(E AND F)

Here P(E AND F) is not given, so the answer is E.

P(E OR F) = P(E) + P(F) - P(E AND F) is true if E and F are dependent events.However if they are independent Events P(E OR F) = P(E) + P(F) .Question does not provide information ..whether E and F are dependent/ independent event ...so it should be E

Re: : What is the probability that event E or event F or both w [#permalink]

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18 Mar 2011, 09:00

The formula is always true. MGMAT just clarifies the point by seperating the two cases. In fact, we have: P(E AND F) = 0 if E and F are independent. _________________

Re: : What is the probability that event E or event F or both w [#permalink]

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18 Mar 2011, 11:18

Quote:

P(E AND F) = 0 if E and F are independent.

Not true, P(E AND F) = 0 if E and F are MUTUALLY EXCLUSIVE.

To say that E and F are independent means only that the outcome of E has no effect on the outcome of F and vice versa.

Quote:

P(E OR F) = P(E) + P(F) - P(E AND F) is true if E and F are dependent events.However if they are independent Events P(E OR F) = P(E) + P(F)

If they are independent events you still need the -P(E and F)

For example: If you flip two coins (coins 1 and 2) at the same time, and we call the event that coin 1 lands heads "E" and the event the coin 2 lands heads "F", what is P(E or F)?

Ifor example, if two coins are flipped the chance of both being heads is [18]

Mutually exclusive If either event A or event B or both events occur on a single performance of an experiment this is called the union of the events A and B denoted as .

For example, the chance of rolling a 1 or 2 on a six-sided die is

This is from Wikipedia. I found this when I was searching for the explanation for this question. But I am still confused the difference between independent probability and mutually exclusive. To me, they sound the same. Help me understand them.

Re: What is the probability that event E or event F or both will [#permalink]

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14 Jan 2014, 02:39

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Re: What is the probability that event E or event F or both will [#permalink]

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28 Jan 2014, 05:22

1

This post received KUDOS

Mountain14 wrote:

Hi Bunuel,

I am not clear why E is the answer...

we have to find P(E)+P(F) + P(EUF), so using Stm1 and Stm 2, we should be able to find.

i.e 0.6 +0.4 + ( 0.6x0.4)...

So, Isn't the answer should be C

Thanks

I'm not Bunuel but I'm sure I can help

P (A and B) = P(A) * P (B/A). That means the probability of B given that A happens. Now only way that the second term is equal to P(B) is if they are independent events, otherwise the probability will be affected. For instance if you have some balls to pick from and you remove some and don't replace them then successive probabilities will change right? So back to the question, Are they independent events? We just don't know this.

Hope it helps brother Cheers! K

PS. Just as a side note check also Mutually exclusive events in the GMAT Club Math Book just to be clear with those Probability concepts

Re: What is the probability that event E or event F or both will [#permalink]

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28 Jan 2014, 06:34

Hi we have no information weather E and F are independent events. So we cant answer the question even by considering the two choices So ans should be E

Re: What is the probability that event E or event F or both will [#permalink]

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10 Mar 2015, 02:44

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: What is the probability that event E or event F or both will [#permalink]

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17 Mar 2016, 18:28

banksy wrote:

What is the probability that event E or event F or both will occur?

(1) The probability that event E will occur is 0.6. (2) The probability that event F will occur is 0.4.

we are asked P(E or F) + P(E and B) usually, if the two events are mutually exclusive, then: P(E) + P(F)=1. - and P(E or F)=1, and P(E and F)=0. if not, then: P(E or F) = P(E) + P(F) + P(E and F) and P(E or F)+P(E and F) = P(E) + P(F)

moreover, it might be conditional probability...

1. P(E)=0.6, but we do not know anything about P(F), or about what kind of events these two are. Moreover, we do not know whether there are any relations between these two. not sufficient.

2. P(F)=0.4 - same as 1. so no

at this moment, we crossed A, B, and D. our chances of getting to the right answer increased considerably - probability 50% (see how I used probability even here? :D)

ok, so it might appear that the two events are independent, since P(E)+P(F)=1. but what if we have a conditional probability? what if F can be chosen only if E is chosen?

in this case, the probability of getting F is P(E)* # of successes/total outcomes left = 4/10 since we do not know the relationship between these two events, then we cannot give a definite answer.

E

gmatclubot

Re: What is the probability that event E or event F or both will
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17 Mar 2016, 18:28

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