00:01
So full disclosure, i am a teacher and i was born and raised in north dakota.
00:08
And i do not teach in north dakota and it might be because of what they're talking about here.
00:13
The average teacher salary in north dakota is $37 ,764 with a population standard deviation of $5 ,100.
00:27
So the first question is what's the probability that a randomly selected teacher's salary is greater than $45 ,000? so we'll do 1 minus the probability that z is less than 45 ,000 minus 37 ,764 divided by 5 ,100.
00:46
And what that does is it gives us a z score.
00:48
So we're looking for 1 minus the probability that z is less than 1 .42.
00:52
Now you don't have to write 1 minus if you don't want to.
00:57
You could just do the probability that z is greater than, but then you're going to have to remember to do that subtraction from 1 at the end of the problem.
01:07
So this probability is equal to .0778.
01:17
Now if there were 75 teachers, that's going to change our standard deviation.
01:26
We'll now use what we call our standard error.
01:29
So our standard error is going to be 5 ,100 divided by the square root of 75.
01:36
And this time we're finding the probability that x is greater than 38 ,000.
01:43
So we'll do 1 minus the probability that z is less than 38 ,000 minus 37 ,764 divided by 5 ,100 divided in turn by the square root of 75.
01:56
So now we have 1 minus the probability that z is less than .40, which corresponds to .3448.
02:09
Now for problem number two, a recent study of the lifetime of cell phones found the average is 24 .3 with a standard deviation of 2 .6.
02:22
A company provides 33 employees with a cell phone.
02:25
Find the probability that the mean life of these phones will be less than 23 .8.
02:32
So for this, we would do our standard error again, which is 2 .6 divided by the square root of 33...