because that is how it sounds
The number 40 is spelled forty.
Sometimes 40 is spelled as forty because it sounds proper. Depending on what you are trying to say, either or could look better with the sentence.
The cardinal number 40 is spelled "forty" and the adjective (ordinal) 40th is "fortieth".
40 dollars is spelled out 'Forty dollars.'
In American English four meaning 4 and forty meaning 40.
The number is spelled "forty" everywhere in English.
The noun form for the number 40 is spelled forty. The plural form is forties.
The correct spelling of the number "14" is, indeed, "fourteen." The number "40," however, is spelled "forty."
The ordinal number 40th is spelled fortieth.
Forty-one Forty-two Forty-three Forty-four Forty-five Note: 4 = Four 14 = Fourteen 40 = Forty as in letter u is omitted in 40
The number 50 is spelled fifty.
Forty (40) has the letters placed in alphabetical order.