00:01
So in this problem, we're going to be working with factorials.
00:05
What is a factorial? whenever you see the exclamation mark here, it's going to mean you multiply the number by every number smaller than it until you get to one.
00:18
So 4 factorial is 4 times 3 times 2 times 1, which is 24.
00:28
7 factorial, same thing.
00:31
Going to be 7 times 6 times 5 times 4 times 3 times 2 times 1 which is equal to 5 ,040.
00:42
So factorials are simple.
00:45
It's just a lot of work.
00:49
Zero factorial.
00:51
So you might think that this is going to be zero.
00:54
However, zero factorial is defined as 1.
01:00
You're always going to have zero factorial.
01:04
Be equal to one.
01:06
The reason being that we use factorials in a probability context, which is why you're doing this right now.
01:14
And if i tell you there's 10 items in a bag, pick zero of them.
01:20
How many outcomes are you going to have? well, that's one outcome, the outcome where you don't do anything.
01:28
It might feel like it's no outcome, but it's really one outcome.
01:32
So that's why zero factorial is equal to 1.
01:39
Now, dividing factorials, this might seem like it's more work, but it's actually a little bit less work.
01:47
So let's write this one out.
01:49
So 6 times 5 times 4 times 3 times 2 times 1 over 2 times 1.
01:56
So we can cancel out a 2 and a 1.
01:59
And we're just left with the 6, 5, 4, and 3, which is equal to 5, 4.
02:04
360.
02:07
So with bigger numbers, this is going to save a lot of time, especially if you don't have a calculator.
02:15
So in this one, we have five times four times three times two times one over two times one times three times one.
02:25
So cancel out the free to one.
02:28
Now we just have five times four divided by 2, which is 10.
02:37
Now, for f, so let's start writing it out, 6 times 5 times 4 times 3 times 2 times 1, over 4 times 3 times 2 times 1.
02:50
So 6 minus 4 factorial, that's 2 factorial, so times 2 times 1.
02:56
And let's start canceling stuff out.
02:59
The 4, 3, 2, and 1 go away, and we're just left with 6 times 5.
03:04
Divided by 2, which is 15.
03:10
Now for g, this is just algebra 1 question that we're being given here.
03:18
This is getting you exposed to the notation for the binomial theorem, but you are just going to multiply point three times itself four times, which is 0 .9 times 0 .9, which is 0 .0081.
03:38
All right...