Why is it that if you add the last 2 digits of your birth year + your age or the age you will become on any particular year it always adds up to 117?
For example, I was born in 1975 and turned 42 this year so: 75+42=117
My dad was born on 1947 and turned 70 this year, so: 47+70=117.
My brother was born on 1985 and turned 32 this year: 85+32=117
Why is it always 117 for anyone born in the 1900's?
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Alex
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3because this is the year of 2017? I bet the number will be 118 next year. – Vasili Oct 26 '17 at 19:53
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1because birth year + age = current year, which is 2017. – Wouter Oct 26 '17 at 19:53
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1That is because 1900+117=2017.. – yngabl Oct 26 '17 at 19:55
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"In any particular year" is wrong. It's only true if you give you start with you age on Dec. 31, 2017 and you were born before 2000. – Thomas Andrews Oct 26 '17 at 20:07
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It is now $2017$, which is $1900+117$. If you were born in year $y$ of the $1900$s the last two digits of your birth year is $y-1900$ and your age now is $2017-y$. Adding these gives $117$
Ross Millikan
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The definition of age (as long as you've had your birthday this year) is $$ \text{Age} = \text{Current year} - \text{Birth year} $$ Rearranging, we get $$ \text{Age} + \text{Birth year} = \text{Current year} $$ which is what you have observed. You're off by $1900$ (i.e. get $117$ instead of $2017$) because you're using only the last two digits of your birth year, which for people born in the twentieth century in practice means that you've subtracted $1900$ from the left-hand side of the last equation.
Arthur
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1In particular, it doesn't work for anybody born in 2000 or later - then the result you get is $17$. – Thomas Andrews Oct 26 '17 at 20:06