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.

_{4}C_{3} = | 4! |

3!(4 - 3)! |

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

n! = 4!

4! = 4 x 3 x 2 x 1

4! = 24

(n - r)! = (4 - 3)!

(4 - 3)! = 1!

1! = 1

1! = 1

r! = 3!

3! = 3 x 2 x 1

3! = 6

_{4}C_{3} = | 24 |

6 x 1 |

_{4}C_{3} = | 24 |

6 |