Correct spelling, explanation: fifteen is the correct spelling because of the origin of the word five. The last f in the original word fif became voiced. However, in fifteen this didn’t occur because t is an unvoiced consonant. To create numbers from 13 to 19 we add the suffix -teen to the cardinal numbers, therefore fifthteen is incorrect because the suffix is added to the ordinal number.

Definition of fifteen:
numeral, a number between 14 and 16
You can’t drink alcohol when you’re only fifteen!
James has visited fifteen countries in Europe so far.

Incorrect spelling, explanation: fifth is an ordinal number and we add the suffix -teen to cardinal ones, therefore spelling fifthteen is incorrect. The form fifteen is correct because the word five used to be unvoiced, which means the original form was fif and this is the form we use now to create the form fifteen.

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling