Forty looks strange because it comes from four, but standard English spells the number without the letter u. So the correct form is forty, not fourty.
No one can point to one clear reason the spelling changed. In earlier stages of English, spelling was much less fixed than it is now. Over time, forty became the accepted written form, and that version stayed.
What makes it feel inconsistent is that related words still keep the u:
- four
- fourteen
- fourth
But when you write the number 40 or numbers built from it, use forty:
- forty students
- forty pages
- forty two
This is one of those English spellings that follows usage more than logic. If you remember just one contrast, make it this: four, but forty.
