let w, x, y, and z represent the probabilities of the regions
they are in in the Venn diagram below
P(A and B) = "what's in both circles" = x
P(A and Cc) = "what's in A but not in B" = w,
P(Ac and B) = "what's not in A but is in B" = y,
P(Ac and Bc) = "what's not in A and not in B" = z
Simplifying the third and fourth equations:
We have this system:
Solve the system:
and get w = 0.4218, x = 0.3182
Solve the system:
and get y = 0.169, x = 0.091
Put those values in the Venn diagram:
Edwin
.
(1) To calculate P(A and B), it is enough to have these two given values P(A)= 0.74 and P(B | A)= 0.43.
By the definition of the conditional probability, P(B | A) = P(A and B)/P(A).
Therefore, to get P(A and B), simply multiply P(A | B) and P(A) to get
P(A and B) = 0.74*0.43 = 0.3182. ANSWER
(2) P(A and Bc) = P(A) - P(A and B) = 0.74 - 0.3182 = 0.4218. ANSWER
(3) The solution for (3) is absolutely identical to the solution for (1).
Therefore, I will repeat it word-in-word.
To calculate P(Ac and B), it is enough to have these two given values P(Ac)= 0.26 and P(B | Ac)= 0.65.
By the definition of the conditional probability, P(B | Ac) = P(B and Ac)/P(Ac).
Therefore, to get P(Ac and B), simply multiply P(B | Ac) and P(Ac) to get
P(Ac and B) = 0.65*0.26 = 0.169. ANSWER
(4) P(Ac and Bc) = P(Ac) - P(Ac and B) = 0.26 - 0.169 = 0.091. ANSWER
Solved.
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Thus, to solve the problem
- you don't need to use and to plot Venn diagram;
- you do not need to form and to solve the system/systems of linear equations.
All you need is to know the basic definitions and properties of probabilities (and of conditional probability, in particular).
In this context, the Edwin' solution leads you to NOWHERE.
This problem is of introductory level on conditional probability, and the solution technique must be adequate.
I am 200% sure that this problem was designed, created, intended and expected to be solved by the method of my post.
I write it to you not for making a war with Edwin, but with the unique goal to show you the correct way solving such problems.
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On conditional probabilities, see the lesson
- Conditional probability problems
in this site.