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.

_{8}C_{2} = | 8! |

2!(8 - 2)! |

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

n! = 8!

8! = 8 x 7 x 6 x 5 x 4 x 3 x 2 x 1

8! = 40,320

(n - r)! = (8 - 2)!

(8 - 2)! = 6!

6! = 6 x 5 x 4 x 3 x 2 x 1

6! = 720

r! = 2!

2! = 2 x 1

2! = 2

_{8}C_{2} = | 40,320 |

2 x 720 |

_{8}C_{2} = | 40,320 |

1,440 |