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!
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.