The formula for a combination of choosing

_{n}C_{r} = | n! |

r!(n - r)! |

where n is the number of items and r is the unique arrangements.

_{3}C_{3} = | 3! |

3!(3 - 3)! |

Remember from our factorial lesson that n! = n * (n - 1) * (n - 2) * .... * 2 * 1

n! = 3!

3! = 3 x 2 x 1

3! = 6

(n - r)! = (3 - 3)!

(3 - 3)! = 0!

0! =

0! = 1

r! = 3!

3! = 3 x 2 x 1

3! = 6

_{3}C_{3} = | 6 |

6 x 1 |

_{3}C_{3} = | 6 |

6 |