40

40 is an integer that follows 39 and precedes 41. In decimal form it is written as 40. It has representations in several numeral systems: binary 101000, octal 50, decimal 40, hexadecimal 28, and Roman numerals XL. In base 3 it is 1111.

Mathematically, 40 factors as 2^3 × 5. It has eight divisors: 1, 2, 4, 5, 8, 10, 20, 40. The sum of its divisors is 90, so the sum of proper divisors is 50; therefore 40 is an abundant number.

As a base-10 number, 40 is a Harshad (Niven) number because it is divisible by the sum of its digits (4 + 0 = 4). It is not prime.

In science, 40 is the atomic number of zirconium (Zr), a transition metal used for ceramics, alloys, and nuclear applications.

Culturally and in timekeeping, 40 appears in biblical contexts such as forty days and forty nights, in the concept of Lent, and as a rough gestation period of about 40 weeks in human pregnancy.

In numeral curiosities, 40 in base 3 is 1111_3; in base 8 it is 50_8; in base 16 it is 28_16.