# TI-82: Counting Principles

## Factorials

The factorial (!) key is located under the math probability menu. Enter the number first, then the factorial key.

```   10! = 10 MATH PRB 4
```

## Permutations

The permutation key (nPr) is located under the math probability menu. Enter the number of objects, n, first; then the permutation key; then the number of objects to take at one time, r.

```P(10,3) = 10P3 = 10  MATH PRB 2  3
```

## Combinations

The combination key (nCr) is located under the math probability menu. Enter the number of objects, n, first; then the combination key; then the number of objects to take at one time, r.

```C(10,5) = 10C5 = 10  MATH PRB 3  5
```

Sometimes, combinations need combined with the fundamental counting principle. This can easily be done one the calculator.

Example: How many ways can five women be selected from ten women and three men selected from eight men? The solution is shown below. The parentheses are optional, but it is suggested you use them for clarification

```   ( 10 nCr 5 ) * ( 8 nCr 3 )
```