Summation
Riddle: This number added to it's square, and the digits of that summation added together, bring it back to itself. What is it?
Answer: 3. 3 squared is 9. 3+9=12. Take the two digits of the summation and add them; 1+2=3. In short; 3-9-12-3. This can also work with the number 0 and 9.
Source: https://www.riddles.com/3770
