Math Magic With The Number 9

Sent me by Rick Gordon:


• First of all, pick the number of times a week that you would like to have chocolate (more than once but less than 10).

• Multiply this number by 2 (just to be bold).

• Add 5.

• Multiply it by 50 — I’ll wait while you get the calculator.

• If you have already had your birthday this year add 1759. If you haven’t, add 1758.

• Now subtract the four digit year that you were born.

You should have a three digit number. The first digit of this was your original number (i.e., how many times you want to have chocolate each week).

The next two numbers are:


(This will only work in the year 2009.)

This reminded me of a carpenter’s trick someone showed me recently: take a tape measure and bring the end (tip) back around in a loop so it rests at 109″. Now look up the year you were born on the tape measure, i.e. “56” for 1956. Turn the tape over and opposite the 56 will be your age. What is going on here?

About Lloyd Kahn

Lloyd Kahn started building his own home in the early '60s and went on to publish books showing homeowners how they could build their own homes with their own hands. He got his start in publishing by working as the shelter editor of the Whole Earth Catalog with Stewart Brand in the late '60s. He has since authored six highly-graphic books on homemade building, all of which are interrelated. The books, "The Shelter Library Of Building Books," include Shelter, Shelter II (1978), Home Work (2004), Builders of the Pacific Coast (2008), Tiny Homes (2012), and Tiny Homes on the Move (2014). Lloyd operates from Northern California studio built of recycled lumber, set in the midst of a vegetable garden, and hooked into the world via five Mac computers. You can check out videos (one with over 450,000 views) on Lloyd by doing a search on YouTube:

One Response to Math Magic With The Number 9

  1. Well, it's quite simple if you write it out (I was born in 1972):

    (2x + 5) * 50 + 1759 – 1972
    = 100x + 250 + 1759 – 1972
    = 100x + 2009 – 1972

    so the 100 multiplier brings your mysterious digit into the 3rd place. 2009 is the current year and if you are under 100 the two least significant digits are your age.

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