Can someone write an algebraic equation for this?

[Monkeyleg]

It's been decades since I had to write algebraic equations, and I just can't remember how.

Anywho, a friend emailed me one of those "try this, you'll be amazed" email tricks having to do with there being so many dates this year with 1's in them (1/1/11, 11/11/11, 11/1/11, etc).  Take the year you were born, (let's say 1975), then add the last two digits of that year to your age, and the total comes out to...111.

I explained how it would work for any year, as the last two numbers in the 3-digit string will always be the year. For example, 2005. If you were born in 1980, 80+25=105.

The parlor trick is subtracting the person's age from the current year of the current century (111 being the 11th year following the turn of the 1st century), then adding the remainder of the remaining prior century (92 for example), and arriving back at the same number. I.e., if it's 2008, the number is 108. If you're 25, you're adding 25 to the remainder of the preceding 100 years (83) to come back to 108.

I just can't remember how to write an equation for this. Can someone here with more brain cells please do it?

[just Warren]

http://www.wolframalpha.com/examples/Math.html

[MicroBalrog]

What if I'm ten years old?

[CNYCacher]

It's been decades since I had to write algebraic equations, and I just can't remember how.

Anywho, a friend emailed me one of those "try this, you'll be amazed" email tricks having to do with there being so many dates this year with 1's in them (1/1/11, 11/11/11, 11/1/11, etc).  Take the year you were born, (let's say 1975), then add the last two digits of that year to your age, and the total comes out to...111.

I explained how it would work for any year, as the last two numbers in the 3-digit string will always be the year. For example, 2005. If you were born in 1980, 80+25=105.

The parlor trick is subtracting the person's age from the current year of the current century (111 being the 11th year following the turn of the 1st century), then adding the remainder of the remaining prior century (92 for example), and arriving back at the same number. I.e., if it's 2008, the number is 108. If you're 25, you're adding 25 to the remainder of the preceding 100 years (83) to come back to 108.

I just can't remember how to write an equation for this. Can someone here with more brain cells please do it?

Assuming you are more than 11 years old, and less than 111, like the majority of people reading email, asking someone to take the last two digits of their birth year is a tricky way of asking them to take their birth year and subtract 1900.  That number, plus your current age is always going to be the current year - 1900 which is 111 this year, and was 105 in 2005, etc.

y = current year
a = Age
b = birth year

b - 1900 + a  =  y - 1900
b + a - 1900  =  y - 1900
          b + a  =  y

[280plus]

Wow, it's magic! 

[Monkeyleg]

What if I'm ten years old?

That's where it breaks down (or at least CNYcacher's formula does).