00:01
We have a frequency table here, schools classified by the number of computers that they have.
00:06
We're going to pick a school at random.
00:08
In part, what's the probability? they have 100 or fewer computers, so no more than 100.
00:14
Okay, and i'm going to express this first at a fraction.
00:19
On the denominator, we will have all of the schools.
00:22
On the numerator, the number that meets our criteria.
00:25
This works because each school is equally likely to be chosen.
00:28
So the proportion that meet the criteria is also the probability that our randomly selected school is in that group.
00:36
So how many schools are there in total? so i'm adding up all of the frequencies here, and i get 81 ,404.
00:45
Okay, so for the first part, out of 81 ,4004, how many have 100 or fewer? so that's anything except the 100 plus group.
00:56
So we're going to add those up, or actually what i'm actually.
01:00
Going to do is start at all of them and i'm just going to take away the ones that don't meet my criteria.
01:05
And i know i'll be left ones that do meet my criteria because if i add all of them, i get 81 ,404.
01:12
So let me just take away ones that don't work and we're left with 48 ,190 over 81 ,404.
01:23
So that whole trick would strictly be called the compliments rule.
01:27
If you add up the probability of something happening and the probability does not happen, you get one.
01:31
So if you set at 1 ,81 ,404, over 81 ,404, take away the probability you don't want, that it's greater than 100, you're left for what you do want.
01:42
And turning that into a decimal, that's 0 .592 to 3 decimal places.
01:51
Pop up, more than 20, greater than 20...