# Permutation

How to solve a permutation (nPr): formula, 4 examples, and their solutions.

## Formula

### Formula

Permutation nPr means
start from n and multiply r numbers.
(n ≥ r)

Special values:
nP1 = n
nP0 = 1

## Example 1

### Solution

6P4 is,
start from 6 and multiply 4 numbers,
6⋅5⋅4⋅3.

6⋅5 = 30
4⋅3 = 12

30⋅12 = 360

## Example 2

### Solution

nPr is used
when choosing r things from n things
and arranging them in a row.

It says
choose 3 students from 9 students
and arrange them in a row.

So the number of the ways is
9P3.

9P3 is,
start from 9 and multiply 3 numbers,
9⋅8⋅7.

9⋅8 = 72

72⋅7 = 504

## Example 3

### Solution

There are 8 numbers.

To make a 4-digit number,
choose 4 numbers
and arrange them in a row.

So the number of the ways is
8P4.

8P4 is,
start from 8 and multiply 4 numbers,
8⋅7⋅6⋅5.

8⋅7 = 56
6⋅5 = 30

56⋅30 = 1680

## Example 4

### Solution

There are 6 numbers.

To make a 3-digit number,
choose 3 numbers
and arrange them in a row.

The number of the ways is
6P3.

But there's a case to subtract.

If 0 is in the hundreds,
then it's not a 3-digit number:
 is not a 3-digit number.

So you should subtract this case.

If 0 is in the hundreads,
there are 5 numbers to choose,
choose 2 numbers,
and arrange them in tens and ones.

The number of the ways to subtract is
5P2.

So the number of ways
to make a 3-digit number is
6P3 - 5P2.

6P3 is,
start from 6 and multiply 3 numbers,
6⋅5⋅4.

Minus,
5P2 is,
start from 5 and multiply 2 numbers,
5⋅4.

So 6P3 - 5P2
= 6⋅5⋅4 - 5⋅4.

6⋅5⋅4 - 5⋅4
= 5⋅4(6 - 1)

Common Monomial Factor

5⋅4 = 20
6 - 1 = 5

20⋅5 = 100