Why is the figure 40 spelt forty wthout a u when the u is used in 4, 14, 400 etc?

Question

Why is the figure 40 spelt forty wthout a u when the u is used in 4, 14, 400 etc?

Answer 1

With English there doesn't always have to be a reason. Sometimes words just evolve. 'Forty' is one of them, and 'fifty' another.

Answer 2

It did!  Etymology:

    Middle English fourty, adjective, from Old English fēowertig, from fēowertig group of 40, from fēower four + -tig group of 10; akin to Old English tÄ«en ten

Date: before 12th century

We tend to shorten up things > but if you wrote "for" that would be different than "four".  Not the case with forty.