00:01
In this question, we're given a set a has six elements.
00:05
Now, by the definition of sets, all elements are distinct.
00:15
That means there are no repeats of element inside a set.
00:23
So, for example, if a has these six elements, a, b, c, d, a, is not equal to b, not to c, not equals to d, not equals to e, not equals to f.
00:44
Now for subsets, the elements chosen are without replacement since they are all distinct, there are no repeats.
00:59
And the order of sets and subsets are not important.
01:08
So for instance, a subset is abc, it's the same as b -a -c, same as c -a -b.
01:20
So the order is not important.
01:29
So we want to find in number one the number of subsets or size exactly 4 that a has.
01:35
So subsets are elements chosen to form the subset without replacement and order not important.
01:44
So we'll be using combination.
01:50
So i have n distinct objects and i choose r of them.
01:54
It will be n factorial divided by r factorial times n minus r factorial...