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555-605 (Medium)|   Probability|                           
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ryjames
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Bunuel chetan2u

What would be the answer if we had this instead: "The probability that event M will occur is 0.8, and the probability that event R will occur is 0.6"?


Then, the statement that both m and n cannot occur together will not be true as 0.8+0.6 will become 1.4>1, which is not possible.

So, the answer would depend on P(Both). And the value could be anything from 0.8 to 1.0 depending on P(Both).
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Bunuel - could you please just clarify one thing.

I understand that we're doing here P(A) + P(B) = 0.6

However, sometimes they'd ask for P(A)*P(Not B) + P(B)*P(Not A). What would be the prompt difference... like "what's the probability that one of the events occurs while the other do not" - that'd be it?

One could argue that you could see "probability that either event M or R will occur" as "Event M occurs AND Event R does not occur OR Event R occurs AND event M does not occur"


Bunuel
The probability that event M will not occur is 0.8 and the probability that event R will not occur is 0.6. If events M and R cannot both occur, which of the following is the probability that either event M or event R will occur?

A) 1/5
B) 2/5
C) 3/5
D) 4/5
E) 12/25
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akwenok
Bunuel - could you please just clarify one thing.

I understand that we're doing here P(A) + P(B) = 0.6

However, sometimes they'd ask for P(A)*P(Not B) + P(B)*P(Not A). What would be the prompt difference... like "what's the probability that one of the events occurs while the other do not" - that'd be it?

One could argue that you could see "probability that either event M or R will occur" as "Event M occurs AND Event R does not occur OR Event R occurs AND event M does not occur"


Bunuel
The probability that event M will not occur is 0.8 and the probability that event R will not occur is 0.6. If events M and R cannot both occur, which of the following is the probability that either event M or event R will occur?

A) 1/5
B) 2/5
C) 3/5
D) 4/5
E) 12/25


We are given that events M and R cannot both occur. Therefore, if A occurs, B does not occur with a probability of 1.
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I know this is old, but in case it helps anyone else understand, if the question DIDN'T say "both cannot occur" (i.e., 100% chance that the other event cannot occur), then the below structure would be right.

However, since it says both cannot occur, we take the probability that one event will occur (either .2 or .4) and multiply it by the second event NOT occurring, which will now always be 1 because the question said there is no way they both can occur.

So the equation becomes (.2)(1) + (.4)(1) = .6 = 3/5
shonakshi
Can someone please explain why is it not
0.8*0.4+0.2*0.6
11/25
:?
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Bunuel
The probability that event M will not occur is 0.8 and the probability that event R will not occur is 0.6. If events M and R cannot both occur, which of the following is the probability that either event M or event R will occur?

A) 1/5
B) 2/5
C) 3/5
D) 4/5
E) 12/25
See This is also an easy question just note donwn the given information
Probability of M not Occur = 0.8
Probability of R not Occur = 0.6

We need M occur or R occur
We add both probability of occur
So for M= 1-0.8=0.2
For R=1-0.6=0.4
Convert them in to fraction:-
2/10 FOR M 4/10 for R now
M+R= 2/10 + 4/10 = 6/10 now we make it small = 3/5
ANS:-C 3/5
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Is it right to use the sum formula here?

Total = A + B - both + neither

So I got: 1 = 0.2 + 0.4 - 0 + Neither
Neither = 0.4

Therefore, I thought, either A or B is the opposite of neither, so 1 - 0.4 = 0.6 or 3/5

My answer is therefore correct but did I do it right? E.g. is it right to assume that Total = 1 ?
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TestTaker0
Is it right to use the sum formula here?

Total = A + B - both + neither

So I got: 1 = 0.2 + 0.4 - 0 + Neither
Neither = 0.4

Therefore, I thought, either A or B is the opposite of neither, so 1 - 0.4 = 0.6 or 3/5

My answer is therefore correct but did I do it right? E.g. is it right to assume that Total = 1 ?
TestTaker0

Yes, your approach is correct. Let me validate your method and explain why it works perfectly for this problem.

Your Formula is Valid

The formula you used (Total = A + B - both + neither) is indeed correct, though it's more commonly written as: \(1 = P(M \text{ only}) + P(R \text{ only}) + P(\text{both}) + P(\text{neither})\)

This is the fundamental principle that all possible outcomes in a probability space must sum to 1.

Why Total = 1 is Correct

Yes, assuming Total = 1 is absolutely right! Here's why:
  • In any probability problem, the sum of all mutually exclusive and exhaustive outcomes equals 1
  • The four scenarios (M only, R only, both M and R, neither M nor R) cover every possible outcome
  • Since these four scenarios account for 100% of possibilities, their probabilities must sum to 1

Alternative Verification:

You could also use the standard addition rule: \(P(M \text{ or } R) = P(M) + P(R) - P(M \text{ and } R) = 0.2 + 0.4 - 0 = 0.6\)
Both methods give the same answer.
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egmat
Thank you very much, this helps a lot!!
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