00:01
Looking at the mean time it takes for a baby to walk.
00:04
And they tell us that the mean time is nine months, and the standard deviation is 1 .5 months.
00:15
And we want to find in part a, and so this would be, here's 10 .5, and down here is going to be 7 .5, one standard deviation away.
00:26
And we want to find the probability that it takes from 8 to 11 months to walk.
00:33
So 8 is going to be about here, and 11 months is going to be about here.
00:38
So we can see that this one is a little over 1 for z value, and this is a little less than negative 1.
00:44
So let's convert them to z values.
00:46
So 8 minus 9 divided by 1 .5, and 11 minus 9 divided by 1 .5.
00:56
So let's get our 2 -place decimal.
00:58
So we have negative 1 divided by 1 .5.
01:01
That comes out to be negative .67.
01:06
It comes negative 6 repeating.
01:08
And this becomes 2 divided by 1 .5, and that comes out to be 1 .3 repeating.
01:15
So the two decimal places, we have that.
01:17
And now we know we find the area below the 1 .33, and that gives me 0 .982.
01:26
And then we find the area below .67 and we subtract those and point six seven corresponds to ziola 0 .2514.
01:38
0 .2514 and then 0 .9082 minus 0 .2514 and that area ends up being 0 .6568 to 4 decimal places.
01:54
And i'm going to do one just real fast double check, make sure i didn't make a mistake in my calculations, and negative 1 .33, oops, negative .6, 7.
02:07
I'm kind of using a normal cdf double to double check to make sure i didn't make a mistake in subtracting.
02:14
And i'm getting the answer to be slightly different.
02:18
Oops, that's because i hit it in wrong...