00:01
So in this question we have that x is the number shown on a red fair dye, and y is equal to zero if the two dice share the same number.
00:26
It's equal to one if the number on the green die is bigger than the number on the red die.
00:34
But the number on the green die, the number on the red dye is x.
00:38
And it's two if the number on the red dye is greater than the number on the green die.
00:45
So what we can say, so let's write down a joint probability distribution.
00:52
So x can be 1, 2, 3, 4, 5 or 6, and y can be 0, 1 or 2.
01:10
So let's draw out this table.
01:16
Now let's fill it in.
01:17
So if x is equal to 1, well the probability y is 0 is always going to be the same, because the probability that the two of them are equal is always going to be 1 6th given each value of x.
01:32
So that means that multiplying that by a 6th, we get 1 over 36 for each, that y will be 0, and that x will show any particular number.
01:43
So this is 1 over 36, 1 over 36.
01:49
Etc.
01:55
Now for y to be one, that means the number on the green dye has to be bigger than x.
02:00
So for x equals 1, there are five numbers that are bigger than x.
02:06
So, and then the probability that x is each of these numbers is going to be one -sixth.
02:10
So we get five -six times one -sixth.
02:12
That gives us five over 36.
02:15
Here we have four six, because there's four numbers that are bigger than x, and we have a one -sixth chance of x being two.
02:21
So we have four over 36, three, three, over 36, 2 over 36, 1 over 36, and then 0, because if x is 6, there's no way that the green dye can be bigger than that.
02:35
But then if x is bigger than the green die, well, if x is 1, that can't happen.
02:40
If x is 2, then we've got a 1 over 36 chance of that being of this situation occurring.
02:46
And this is just going to look like the mirror image of the line above.
02:54
And now let's see whether these all add up to 1, because that will tell us, give us a check of whether this all works out...