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40...

40 (forty) is the natural number following 39 and preceding 41.

Though the word is related to "four" (4), the spelling "forty" replaced "fourty" in the course of the 17th century and is now the standard form.

Forty is a composite number, an octagonal number, and as the sum of the first four pentagonal numbers, it is a pentagonal pyramidal number. Adding up some subsets of its divisors (e.g., 1, 4, 5, 10 and 20) gives 40, hence 40 is a semiperfect number.
Given 40, the Mertens function returns 0. 40 is the smallest number n with exactly 9 solutions to the equation (x) = n.
Forty is the number of n-queens problem solutions for n = 7.
40 is a repdigit in base 3 (1111, i.e. 30 + 31 + 32 + 33) and a Harshad number in base 10.

Are you curious? Keep reading here: https://en.wikipedia.org/wiki/40_(number)