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JustJessica's avatar

Why does this happen? (details inside)

Asked by JustJessica (4054points) January 19th, 2011

On facebook today I read this status : This year we will experience 4 unusual dates…. 1/1/11, 1/11/11, 11/1/11, 11/11/11…. NOW go figure this out…. take the last 2 digits of the year you were born plus the age you will be THIS YEAR and it WILL EQUAL…. 111!!!

I did this with 20 different birthdays and they all equaled 111.
Why is this?
Is their some mathematical explanation?
Is it something simple that I just keep missing?

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17 Answers

mowens's avatar

Mine was 110.


Born 1983 and I will be 27 this year. 110.

Crap I turned 27 two months ago… it does equal 111. Wtf.

mowens's avatar

My grandma was born in 1910, and would be 110 this year.

120. :) Had to cheat.

mowens's avatar

because it is going from the same relitave point. You start at 100.

Brian1946's avatar

I think the reason for that is because the last two digits of the year you were born, plus the age you will be this year, have to add up to this year.

E.g., if you asked this question in 2010, the last two digits of a birth year before 2000, plus the age reached in 2010, would add up to 110.

I think the results are all greater than 100 for anyone born before 2000 because those calculations span two centuries.

If you calculate for someone born in 2006 e.g., then it adds up to 11, and it will for anyone born in this century.

iamthemob's avatar

I think it works if you only consider anyone born in or before 2000 and anyone who will be 100 or younger and it works.

KatawaGrey's avatar

@mowens: Psst, your grandma would be 101 this year. ;)

@Brian1946 is correct, though it is a fun little coincidence. :)

CyanoticWasp's avatar


You are NOT smarter than a fifth-grader. Your grandmother born in 1910 would be 101 years old this year.

Vortico's avatar

Before anyone starts saying “coincidence,” remember that the references of dates and the base 10 is arbitrary. The number of seconds since midnight January 1, 1970 in base 36 right now happens to be LFAHW8!

ratboy's avatar

If you were born between 1900 and 1999, then:
your age on your birthday this year will be 2011 – (1900 + d), where d is given by the last two digits of the year you were born. Since 2011 – (1900 + d) = (2011 – 1900) – d, when you add your age this year to the last two digits of the year you were born the result is (2011 – 1900) – d + d = 2011 – 1900 = 111.

PhiNotPi's avatar

The reason that this works is that as the year you were born in decreases, your current age increases. If you were born one year earlier than someone, you will be one year older than that person. When you add these numbers together, the decrease of one number and the increase of the other cancel out perfectly. As an extension to ratboy’s answer, the same idea would mean that in 2012, the numbers would add up to 112.

iamthemob's avatar


I think your equation is off.

2011 – (1900 + d) = 2011 – 1900 – d, which would mean that the equation is simplified to 111 – d. The transitive property means you multiply the two numbers in the parentheses by (-1). You lose the parentheses then – but even if you use them as you do, it doesn’t matter.

The age you are this year, also, is not equal to d. It should be equal to 111 – d as well.

So the equation is really 111 – d + (111 – d). Which doesn’t work out…

PhiNotPi's avatar

Here is the correct equation- the two digit number is d (the birth year is 1900+d), and the age is 2011—(birth date, which is 1900+d). When you add these together, you end up with 2011—(1900+d)+d, which simplifies to 2011–1900, or 111. @ratboy did have the correct equation, he just explained it wrong.

ratboy's avatar

@iamthemob, the property you mention is the distributive property (of multiplication over addition). It doesn’t matter where the parentheses are placed; I placed them as I did for emphasis.

“The age you are this year, also, is not equal to d. It should be equal to 111 – d as well.”

your age on your birthday this year will be 2011 – (1900 + d), where d is given by the last two digits of the year you were born.

You’re right that the equation for your age this year simplifies to “111 – d”, and it is to this that d (the last two digits of the year you were born) is to be added. The number 111 is the number of years from 1900 to 2000 plus the number of years since 2000. The last two digits of the year you were born is the number of years between 1900 and the year of your birth.

iamthemob's avatar


Crapballs. You’re right on the distributive property.

smchilds's avatar

Since the year 1900 there have been 111 years. Of course, the years before you were born (beginning with 1900) and the years after you were born will add up to 111 years. It is pretty simple. Of course next year everyone’s age + birth year will equal 112. Then 113. Because the birth year and age account for all years since 1900. It is just a coincidence that this is also the year with 1/1 1/11 11/1 and etc.

Brian1946's avatar

Here’s another simplified perspective:

The year a person was born plus the age they will be in 2011=2011.
Sample equation using my birth year: 1946+65=2011.

To take just the last 2 digits of my birth year, subtract 1900 from my birth year.
To balance the above equation so that it stays correct, also subtract 1900 from the other side so that 46+65=111.

JustJessica's avatar

Woo Hoo Thanks everyone, gotta love a good ole fashion math battle! Oh I hate math!

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