Have a person mark any given calendar month in a 3-by-3 “square” containing 9 days. See an example of a calendar choice below for August 2005.

August 2005 | ||||||
---|---|---|---|---|---|---|

Sun | Mon | Tue | Wed | Thu | Fri | Sat |

1 | 2 | 3 | 4 | 5 | 6 | |

7 | 8 | 9 | 10 | 11 | 12 | 13 |

14 | 15 | 16 | 17 | 18 | 19 | 20 |

21 | 22 | 23 | 24 | 25 | 26 | 27 |

28 | 29 | 30 | 31 |

Then ask her to tell you the smallest date on the square, and say that with just that date you will find out the sum of all chosen dates. To do so, you must add the smallest date (in this case 10) with 8 and multiply the result by 9.

That is, (10 + 8) x 9 = 18 x 9 = 162

(i.e. 10 + 11 + 12 + 17 + 18 + 19 + 24 + 25 + 26 = 162).