forty · number /ˈfɔː.ti/
forty
A2 the number 40
Dictionary examples:
thirty, forty, fifty
He looks about forty.
Learner example:
There were about forty people. (Key English Test; A2; Polish)
(be) in your forties
B2 to be aged between 40 and 49
Dictionary example:
By the time I'm in my forties, I want to be running my own business.
Learner example:
The man was in his early forties and he looked to be a very important man. (First Certificate in English; B2; Basque)
the forties
B2 the years from 1940-1949
Dictionary example:
Most of these people were born in the forties.
Learner example:
So, I would like you to consider buying some English comedies, like those Ealing Studios [made] in the late forties and the fifties. (First Certificate in English; B2; Spanish)