Let x be the age you will turn this year. eg 73
ley y be your birth date, eg 1938
they are related this way:
2011 – y = x
2011 – 1938 = 73

"Add the age you will turn this year, to the last 2 digits of the year you were born"
To get the last two digits, subtract 1900 from your birth year
x + y – 1900
substitute x from above
2011 – y + y – 1900
111

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