00:01
We are given a mean of 2 ,800 and a standard deviation of 860.
00:09
And we're going to need to use the z score formula, which is x minus mu over sigma.
00:16
X is our value that we're interested in.
00:21
So for a, we want to find the probability that there are 2 ,000 or fewer downloads of amazon alexa in a day.
00:28
So that's the probability that x is less than equal to 2 ,000.
00:33
So we need a z score for 2 ,000.
00:36
2 ,000 minus the mean over the standard deviation.
00:41
It gives us a z score of negative 0 .93.
00:54
And if you're using a table, you're going to use negative 0 .93, because it reads to the left.
01:15
And to four places that would be .1762 or if you are using a calculator instead of a table then it might be slightly off yeah just slightly 0 .1761 using a calculator what i've boxed is if you were to use a table for part b, the probability between 1 ,500 and 2 ,500.
02:10
So we need z scores for both, 1 ,500, and 2 ,500.
02:19
So for 1 ,500, and for 2 ,500, for 1 ,500, the z score is about negative 1 .51, and for 2 ,500, it is about negative 0 .31 .1, and for 2 ,500, it is about negative 0 .35.
02:47
Most tables use just two decimal places.
02:50
So this is equivalent to the probability that a z score is between negative 1 .51 and negative 0 .35, which is the same thing as the probability that z is less than a negative .35 minus the probably that z is less than negative 1 .51.
03:13
And if you're using a table, then when you look up negative .35, you'll get .3...