To determine the probability of selecting a subject who is not pregnant, given that the test result is positive, follow these steps:
1. Identify Given Data from the Table:
- Number of subjects who are pregnant and have a positive test result: 78
- Number of subjects who are pregnant and have a negative test result: 7
- Number of subjects who are not pregnant and have a positive test result: 12
- Number of subjects who are not pregnant and have a negative test result: 28
2. Calculate the Total Number of Positive Test Results:
- Add the number of positive test results for pregnant subjects (78) and the number of positive test results for not pregnant subjects (12).
[tex]\[
\text{Total number of positive test results} = 78 + 12 = 90
\][/tex]
3. Calculate the Probability:
- The probability of a subject being not pregnant given the test result is positive is the ratio of the number of not pregnant subjects with a positive test result to the total number of positive test results.
[tex]\[
\text{Probability} = \frac{\text{Number of not pregnant subjects with positive test}}{\text{Total number of positive test results}} = \frac{12}{90}
\][/tex]
4. Simplify the Probability:
- Simplify the fraction.
[tex]\[
\frac{12}{90} = \frac{2}{15} \approx 0.1333
\][/tex]
5. Compare the Result to the Provided Options:
- The result [tex]\(0.1333\)[/tex] closely matches answer choice B, which is 0.13.
Thus, the correct answer is:
B. 0.13
