00:01
We are told that on average a textbook author makes two word processing errors per page.
00:11
And then we're asked for the probabilities that on the next page, so we'll make these numbers of errors.
00:18
Now when we have some phenomenon such as errors in a textbook occurring randomly, and all we have is an average rate at which they occur, we can model the number of occurrences in a given duration.
00:34
Or in this case in a given number of pages, as a plusan random variable.
00:40
So let's define a random variable x as the number of errors in a given page.
00:56
X can be modeled as a pluson random variable with a mean of two.
01:14
This is a mean of two errors per page.
01:19
Now the probability mass function for the pluson random variable is given by this formula.
01:36
And so for our situation, this is 2 to the exponent x times e to the minus 2 over x factorial.
01:49
And for a we want the probability that we have 4 or more errors.
01:54
This is the probability that x is greater than it are equal to 4.
01:59
And this can be re -expressed as 1 minus the probability that x is at most 3.
02:10
Now the probability that x is at most 3 is equal to the probability that x equals 0, plus the probability of x equals 1, and so on up to x equals 3...