Question

You take a wrong turn on the way to Port Blair and end up in Forks, where 99% of the inhabitants are vampires and the rest are regular humans. For obvious reasons, you want to be able to figure out who's who. On an average, nine-tenths of the vampires are correctly identified as vampires and nine-tenths of humans are correct identified as humans. What is the probability that someone identified as a human is actually a human?

• A. 9/1000
• B. 1/12
• C. 1/26
• D. 1/9

Answer 1

The correct option is B

1/12

The probability is taken as : (the number of humans who were correctly identified as humans)/(the number of vampires incorrectly identified as humans)+(the number of humans who were correctly identified as humans)
$=\frac{0.01\times 0.9}{0.99\times 0.1+0.01\times 0.9}$
$=\frac{1}{12}$