The number 40 is spelled forty. This is true in all main varieties of modern English. Fourty is a surprisingly common misspelling that appears most often in the spelling of compound numbers such as 41 (misspelled fourty-one), 42, and so on—for example:
Fourty-five percent of Australians strongly agreed with the value proposition and two percent strongly disagreed. [RenewableEnergyWorld.com]
Fourty-nine stocks depreciated in price during the week, lower than the 58 of the preceding week. [Vanguard]
In a decade we’ll scream to the heavens asking why we paid fourty dollars for a dozen cupcakes, the same way people question their sanity after a flock of seagulls’ haircut. [Washington Examiner]
Fourty-two percent of voters want no cuts in programs for the poor. A year ago, that number was 51 percent. [NJ.com]
The misspelling is understandable, as the first syllable of the word sounds the same as four and the first syllable in fourteen, and there’s no good reason that forty should be different. It is so merely by convention. And though forty has been standard for a few centuries, fourty appeared much more often several centuries ago, before many of our modern spellings were settled.
Fortieth likewise does not have a u.