Another GRE study question
Let A, B, C, and D be events for which P(A or B) = 0.6, P(A) = 0.2, P(C or D) = 0.6,and P(C) = 0.5. The events A and B are mutually exclusive, and the events C and D are independent.
Part (a) asks find P(B), which is
$P(A \cup B) = P(A) + P(B)$ $0.6 = 0.2 + P(B)$ $P(B) = 0.4$
But part (b) asks find P(D), and when I try, my answer is $0.1$
$P(D) = P(C\cup D) - P(C) = 0.6 - 0.5 = 0.1$
This is incorrect. According to the study guide, answer is $0.2$
Please explain
$\endgroup$ 13 Answers
$\begingroup$$$P(C\cup D) = P(C)+P(D)-P(C\cap D)$$
$$0.6 = 0.5 +P(D) - P(C).P(D) = 0.5 +P(D) - 0.5P(D)$$
$$.5P(D) = .1 $$
$$P(D) = .2$$
The catch is Cand D are independent, then $P(C\cap D$ = P(C).P(D)
$\endgroup$ 3 $\begingroup$a) OR$=\cup$, AND$=\cap$ Since $A$ and $B$ are disjoint (mutually exclusive), then $$\{A\cap B\} = \{AB\} = \varnothing.$$ Thus $$P(AB) = P(\varnothing) = 0.$$ Recall the inclusion-exclusion rule $$P(A\cup B) = P(A)+P(B) - P(AB).$$ This implies $$P(B) = P(A\cup B)-P(A)+P(AB) = .6-.2+0 = .4$$
b) Since, $C$ and $D$ are independent, $$P(CD) = P(C)P(D).$$ Again, by inclusion-exclusion, \begin{align*} P(D) &= P(C\cup D) -P(C)+P(CD)\\ &= P(C\cup D)-P(C)+P(C)P(D), \end{align*} and combining like terms yields $$P(D)-P(C)P(D) = P(D)[1-P(C)] = P(C\cup D)-P(C).$$ Solving for $P(D)$ gives $$P(D) = \frac{P(C\cup D)-P(C)}{1-P(C)} = \frac{.6-.5}{1-.5} = .2.$$
$\endgroup$ $\begingroup$You can't say $P(D)=P(C$ or $D)-P(C)$ because $C$ and $D$ are not mutually exclusive. Independent implies that they are not mutually exclusive.
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