Let "P=" the event ''John decides to choose Physics'', "C=" the event ''John decides to choose Chemistry'', "B=" the event ''John decides to choose Biology.''
Given that
"P(C|P)=0.8, P(B|P)=0.75"
"P(C|B^C)=0.6, P(C\\cap P^C|B)=0.35"
Conditional probability
If John has decided to choose Physics, the probability of choosing Biology
If John has decided to choose Physics, the probability of choosing Chemistry
"P(C|P)={P(C\\cap P) \\over P(P)}=>P(C\\cap P)=P(C| P)P(P)""P(C\\cap P)=0.8\\cdot0.4=0.32"If John has decided not to choose Biology, the probability of choosing Chemistry
If John has decided to choose Biology, the probability of choosing Chemistry but not Physics
"=>P(C\\cap P^C\\cap B)=P(C\\cap P^C|B)P(B)"
"P(C\\cap P^C\\cap B)=0.35\\cdot0.6=0.21"
(a) What is the probability that John will choose all three courses?
(b) What is the probability that John will not choose any course in the summer semester?
"P(P\\cap C^C\\cap B)=P(P\\cap B)-P(P\\cap C\\cap B)=""=0.3-0.25=0.05"
"P(P\\cap C\\cap B^C)=P(P\\cap C)-P(P\\cap C\\cap B)=""=0.32-0.25=0.07"
"P(C\\cap P^C\\cap B)=0.21"
"P(P \\cup B\\cup C)=P(A)+P(B)+P(C)-""-P(P\\cap C^C\\cap B)-P(P\\cap C\\cap B^C)-""-P(C\\cap P^C\\cap B)-2P(P\\cap C\\cap B)"
"P(P \\cup B\\cup C)=0.4+0.6+0.7-""-0.05-0.07-0.21-2\\cdot0.25=0.87"
"P(no \\ course)=1-P(P\\cup B \\cup C)=""=1-0.87=0.13"
The probability that John will not choose any course in the summer semester is "0.13"
(c) If John has decided to choose Chemistry, what is the probability that John will choose exactly two courses in the summer semester?
"={0.07+0.21\\over 0.7}=0.4"
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