00:01
Problem we want to select six students from a class of 25 to hold six different positions and that different word is really important because we're not being asked to find groups of six students to form just a group or a committee these six students will be holding different positions perhaps president vice president anyway, that's what distinguishes this problem as what we call a permutation as opposed to just a combination.
00:54
In a permutation, the order that we pick people matters.
00:59
The rule for a permutation is n factorial over n minus k factorial, where n is the total number of people in k.
01:15
And k is the number of positions or slots to fill.
01:23
So for this problem, we have 25 people and we want to take them six at a time in a certain order.
01:35
That's going to give us 25 factorial over 25 minus 6 factorial.
01:47
Over 19 factorial.
01:51
If you remember your factorial notation, for example, 4 factorial, just an exclamation mark after a number, it means start at the number in front of the exclamation point and count down, multiply as you go.
02:09
So 4 factorial is 4 times 3 times 2 times 1 or 24...