Incorrect spelling, explaination: this spelling is incorrect, because we add the suffix -teen to cardinal, not ordinal numbers. Therefore the correct spelling is fifteen (although the number we add the suffix to is spelled five, the second f in fifteen is not voiced, because there is unvoiced consonant following directly).
Correct spelling, explaination: the correct form of this cardinal number is fifteen. It might be confusing that five became fif after adding the suffix, however the word five used to be, originally, fif. The second f in the word became voiced with time. In the word fifteen the f wasn’t voiced, because it is followed by unvoiced consonant t.
Fiftheen is incorrect, because the suffix -teen is added to cardinal numbers, not ordinal ones.
Definition of fifteen:
numeral, between fourteen and sixteen
She got a beautiful bouquet of fifteen tulips from her husband for their 1st anniversary.
Betty is only fifteen but she has a part-time job already.
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