Question: The probability that a randomly chosen record has an error is 0.05. An error in a record is either large or small. Two out of ten errors, on average, are large.
a. What is the probability that a randomly chosen record has a large error
Using Wolfram Mathematica, my sample space is
Since the probability of any record having an error is 0.05 with $\frac{2}{10}$ being a large error, it must be true that
$\frac{2}{10}$ $\cdot$ 0.05= 0.01 is a large error and the probability of a small error is 0.99.
b. Given that a randomly chosen record does not have a large error, what is the probability that it does not have an error.
b. is a conditional probability.
In my attempt, we have
$P\left ( Null Error | Small Error \right )=\frac{P\left ( Null Error \cap Small Error \right )}{P\left ( Small Error \right )} = \frac{0.99}{0.76}$
But this looks odd. It doesn't make sense for me to talk of small or large error in the null error case.
Any help is appreciated.