This is more of a trick than a random fact but now I will be able to guess your age:

1. Choose a number between 1 to 9. Any.

2. Double that number from step 1 and add it to 5.

3. Multiply your new total by 50.

3. Add this number (from step 3) to 1769 (if you celebrated your birthday this year) else 1768.

4. Subtract your birth year (e.g. 2004) from your value that you got in step 3.

5. The first number of the final three digit value is the number you chose. The last two digits are your age.

Is it pretty cool??

1. Start with the number \(n\), where \(n\) is an integer between 1 and 9 (inclusive).

2. New number: \(2n+5\)

3. New number: \(100n + 250\)

4. I'll use 1769 since I have celebrated my birthday. New number: \(100n + 2019\). (Suspicious.)

5. My birth year is 1998. New number: \(100n + 21\).

6. Well the digits of the number are now \(n\), \(2\) and \(1\). So that worked out nicely.

So explanation: That number between 1 and 9 was secretly left untouched since step 3. It just bubbled its way from the units digit to the 100's digit because we literally multiplied it by 100 along the way. Every other operation from there only changed the trailing number.

The trailing number made its way up to a 4-digit number once we added 1769, which was an intentionally rigged choice. This automatically made 2019 appear.

And of course, 2019 (this year) - year of birth = age.

Rule of thumb: Math is more fun when you hide the algebra going on behind the scenes