This is not a mathematical puzzle.
How can 14 become 7?
10 becomes 1, but can also become 2.
13 becomes 8, and 8 becomes 10.
6 stays as 6 under this thinking.
Credit to the series Deltora Quest for this riddle concept. I merely changed one thing about it, and used different numbers. If any of you have read it, you'll get why this is in spoiler tags!
Hint 1:
Don't think of the second in the pair as numbers... perhaps having them in tally form may help you? Some more pairs: 15 becomes 4, as well as 3. 10 can also be 5 or 9, along with 1 or 2 (maybe I should say... "one" or "two"?).
Since answer was found, OP note:
The original puzzle used 11 and 9, and e's were written as "E". As mentioned above, I changed the "E" to a pitchfork shape, eliminating one tally.