Correct spelling

fifteen

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

fifthteen

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 original form was fif and this is the form we use now to create the form fifteen.

Incorrect spelling

fifthten

Incorrect spelling

Incorrect spelling

fiftyn

Incorrect spelling

Incorrect spelling

fiftin

Incorrect spelling

Incorrect spelling

fiveteen

Incorrect spelling

Incorrect spelling

fiften

Incorrect spelling

Incorrect spelling

fivteen

Incorrect spelling