Correct spelling, explanation: this spelling is very often a problem, but it’s easily explainable. Due to pronunciation original fif became voiced and changed to five. F in fifteen however remained unvoiced, because of the way it is pronounced. Therefore fifteen is correct and fiveteen not.

Definition of fifteen:
numeral, the cardinal number between fourteen and sixteen
I can’t believe Julie is already fifteen!
This t-shirt is fifteen pounds.

Incorrect spelling, explanation: linguistically speaking f and v are the same sound, but f is unvoiced and v is a voiced version of it. Originally five was fif, but changed due to pronunciation over the years; fifteen remained with f, because it is followed by another unvoiced consonant (t), so it must be unvoiced too. Therefore fifteen is correct, while fiveteen isn’t.

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling