When it comes to 40 spelling remains the same in case of British as well as American English. 40 is spelled as ‘Forty’ and not ‘fourty’. It may be because of the difference in pronunciation of ‘Four’ and ‘For’ during the evolution of the language. Fortieth also doesn’t contain ‘u’ in it. 14 retains ‘u’ and is spelled as fourteen. Though four, fourteen, fourty appear correct by logic, forty stands separate by dropping u from it saying logic doesn’t apply everywhere and definitely not always in English !!!