Can anyone explain the 111 birthday trick

reply to post by MagoSA


Same as my wife. This doesn't work if your birthday already passed.
She's 37 (now) and born in 74 (January). This would add up to 112 (38+74).

Simple algebra.

The riddle calls for the following:

(birthYear - birthCentury) + ageThisYear = Sum

But, your Age this year can also be written as:

thisYear - birthYear = ageThisYear

Substituing in we get the following:

(birthYear - birthCentury) + (thisYear - birthYear) = Sum

Cancelling out birthYear, we get:

thisYear - birthCentury = Sum

What this means is that everyone born within the same century will always resolve to the same Sum.

If you were born in the 1800's:
2011 - 1800 = 211

If you were born in the 1900's:
2011 - 1900 = 111

If you were born in the 2000's:
2011 - 2000 = 11