Why is 40 spelled "forty" and not "fourty"?

"So four and forty were different words starting a long time ago but were spelled with the same beginning in Old Saxon and Anglo-Saxon and part of Middle English. Somewhere along the way during the Middle English era the simpler spelling of forty took hold and has continued ever since. "

Source:
http://mathforum.org/library/drmath/view/57559.html

To really understand this, you have to go back to the origins of the English language.

In 1066, there was no English at all, nothing even close. You had French (or something like it) and German (or something like it.) The people of what is now England basically spoke German. Then the French invaded and took over, and you had a two language culture; the rulers spoke French, and the ruled spoke German. As time passed, they developed a single language, and not always with perfect consistency. The language was still in early development when the "new world" was discovered, which made even more problems. As uniform spellings were more or less agreed on, there were some exceptions between the different sides of the Atlantic, which is why Americans spell many words without a 'u' such as "color" and "honor", while the English use "colour" and "honour".

If this explains why there are inconsistencies between two English speaking countries, surely it will suffice to explain the inconsistency between "four" and "forty".

answers.yahoo.com/question/index?qid=20071121201919AAOQi3h&show=7 website no longer exists

I think the Yahoo answer is a little misleading in that it suggests there is a difference between the spelling of forty on either side of the Atlantic, as there is with colo(u)r and hono(u)r. It's also a little simplistic to suggest that before the Norman invasion what people spoke in England was something like German. Old English, or Anglo Saxon, would be no easier for a modern German to read than a modern Englishman or American (you might be able to have a better try at it if you speak a Nordic language).

In Old English four, fourteen and forty all came from the same root - feower (meaning four). When you add the equivalent of "ty" to this (which would have been "tig" in old English, meaning "group of ten") that slightly changes the way the preceding vowel is pronounced. Quite early in the development of modern English (and long before the Pilgrim Fathers set sail) this had been reflected in the spelling "forty". The same explanation applies to why "fifty" isn't "fivety".

Sadly, I've no idea why "ninety" isn't similarly altered to "ninty".