i think in spanish that rule doesn't work either... 🙁
(catorce(14) => 7; siete(7) => 5; cinco(5) => cinco.)
(doscientos (200) => diez(10) => cuatro (4) => 6, seis(6) => 4, cuatro...)
Here goes a game you may find interesting (it's quite difficult!!):
You only have to fill in the gaps in the next sentence a way that it becomes true:
"In this sentence the number of times that '1' appears is , the number of times '2' appears is , the number of '3's is , the number of '4's is , the number of '5's is , of '6's is , of '7's is , of '8's is , of '9's is _".
(Note if you start filling: "the number of times '1' appears is 1" the sentence becomes false!.)