# What is a permutation calculation calculating?

it's a calculation that shows the number of ways a group can be picked if we care what the order of those picks is (like the way election results for student government can go).

#### Explanation:

Let's say we have a student government and there are 4 seats to be filled - the President, the Vice President, the Treasurer, and the Secretary. Let's also say there are 4 people running for those seats. How many different ways are there for those 4 people to hold those 4 positions?

We can look at see that for the President's seat, there are 4 candidates.

We can then look at the VP's seat and see that, with someone in the role of President, there are now 3 people left who can be VP.

Then we look to the Treasurer and see there are 2 people left.

And lastly there is the Secretary and the remaining person goes there.

Therefore there are: 4xx3xx2xx1= 4! =24 ways the election results can go.

This is an example of a factoral problem - we're going to take a population and take all the members of that population and place them (whether it's political office or movie seats or whatever - we're using the entire population).

What happens if we have more population than we have seats or places? For example, there are 6 people who are vying for those 4 government slots?

We could do it this way:

We can look at see that for the President's seat, there are 6 candidates.

We can then look at the VP's seat and see that, with someone in the role of President, there are now 5 people left who can be VP.

Then we look to the Treasurer and see there are 4 people left.

And lastly there is the Secretary and there are 3 remaining people who could go there.

So we get $6 \times 5 \times 4 \times 3 = 30 \times 12 = 360$ ways to do this. But this is awkward to do all the time - especially if we have a large population and a large number of places that need filling (ex. A concert hall has 1,000 seats. What is the number of ways 5,000 fans can sit in those 1,000 seats). Wouldn't it be great if there were a formula that would make it easier to work with these types of calculations?

Notice that in the "6 people, 4 offices" example, we ended up with this calculation:

$6 \times 5 \times 4 \times 3$

How can we put that in terms of factorials? Well, if we multiply it by $2 \times 1$ (and divide by that figure as well), we get:

$\frac{6 \times 5 \times 4 \times 3 \times 2 \times 1}{2 \times 1}$

And keep in mind that the definition of a factorial is that we multiply by each natural number up to and including the one with the sign (so 6! = 6xx5xx4xx3xx2xx1), we can express this as:

(6!)/(2!)

And now see that the number 6 is the number of our population while 2 is the difference between the population and the number of seats that needed to be filled, $6 - 4 = 2$. So we can express this as:

(6!)/((6-4)!)

And that is exactly what the permutation formula is! If I call $n$ the population and $k$ the number I'm going to pick, we get:

P_(n,k)=(n!)/((n-k)!)

Lastly, I want to show that this formula still works if we have that initial example - 4 people running for 4 seats - we should get 4!:

P_(4,4)=(4!)/((4-4)!)=(4!)/(0!)=(4!)/1=4!