Correct spelling, explanation: this is the correct form of this number because teen forms are made by adding such suffixes to numbers from 3 to 9. The spellings are sometimes altered due to orthography rules. In the case of fifteen it is five plus -teen. v in five has to be devoiced since t is also unvoiced, hence fifteen. Fiftheen or any other forms ae incorrect. Fifth is an ordinal number so it doesn’t mash with -teen (the ordinal form of fifteen is fifteenth, cause an ordinal suffix is added to the end).

Definition of fifteen:
numeral, the cardinal number between fourteen and sixteen
– I love your shirt, how much was it? – I don’t remember, around fifteen dollars.
We only have fifteen orders today.

Incorrect spelling, explanation: fiftheen is the incorrect form, such a word doesn’t exist. Fifth is an ordinal number to teen ending wouldn’t be added. The correct form is fifteen – five + -teen. v becomes f because then added to an unvoiced consonant it has to be unvoiced itself. Therefore fiftheen is incorrect and the correct spelling is fifteen.

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling