ANSWER:
0.475
Explanation:
The probability of a person has disease given the test is positive:
P (disease) = 3.8% = 0.038
P (positive | disease) = 93.9% = 0.939
P (positive | no disease) = 4.1% = 0.041
P (no disease) = 100% - 3.8% = 96.2% = 0.962
The probability that the person has the disease given that the test result is positive is calculated as follows:
![\begin{gathered} \text{ P\lparen infected \mid test positive\rparen }=\frac{\text{ P\lparen positive \mid infected\rparen }*\text{ \rbrack P \lparen infected\rparen}}{\text{ P \lparen positive\rparen}} \\ \\ \text{ P \lparen positive \mid infected\rparen }=\text{ P \lparen positive \mid disease\rparen = 0.939} \\ \\ \text{ P \lparen infected\rparen = P \lparen disease\rparen = 0.038} \\ \\ \text{ P \lparen positive\rparen = P \lparen positive \mid infected\rparen }*\text{ P \lparen infected\rparen }+\text{ P \lparen positive \mid no infected\rparen}*\text{ P \lparen no infected\rparen } \\ \\ \text{ P \lparen positive \mid infected\rparen =P \lparen positive \mid no disease\rparen = 0.041} \\ \\ \text{ P \lparen no infected\rparen = P \lparen no disease\rparen = 0.962} \\ \\ \text{ We replacing:} \\ \\ \text{ P \lparen positive\rparen = }0.038\cdot0.939+0.041\cdot0.962=0.075124 \\ \\ \text{ P\lparen infected \mid test positive\rparen }=(0.038\cdot0.939)/(0.075124) \\ \\ \text{ P\lparen infected \mid test positive\rparen = }\:0.47497=0.475 \end{gathered}]()
The correct answer is the first option: 0.475