00:02
We have ourselves this situation.
00:05
We've got a normal distribution with a center at 120 and a standard deviation of 20, as shown in my picture.
00:12
Now what we want to know is if the top, the fastest 10 % are given advanced training, what task time do they need to qualify for that training? well, the fastest 10%, now keep in mind this is time here, so the lower the time, the better.
00:27
So the bottom, the left end of our bell curve here is going to be our answers.
00:33
So we want our area to the left of some marker, some task time down here to give us an area of 0 .10.
00:43
Now, to do that, i'm going to look up in the standard normal probability table, an area of 0 .10, which corresponds to a z score of negative 1 .28.
00:57
Now knowing that z score, now i can come and say what observed value minus the mean divided by the standard deviation gets us to this z score of negative 1 .28.
01:11
So we're going to do a little algebra here...