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_{2} = | 3! |

2!(3 - 2)! |

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 - 2)!

(3 - 2)! = 1!

1! = 1

1! = 1

r! = 2!

2! = 2 x 1

2! = 2

_{3}C_{2} = | 6 |

2 x 1 |

_{3}C_{2} = | 6 |

2 |