40 (forty) is the natural number following thirty-nine and preceeding forty-one.
Its divisors are 1, 2, 4, 5, 8, 10, 20 and 40.
Given 40, the Mertens function returns 0.
Forty is:
- The atomic number of zirconium.
- Significant in Judaic, Christian and Islamic tradition, Forty days and forty nights being the period for which rain fell during Noah's flood, the period Moses was on Mount Sinai receiving the 10 commandments and the period Christ spent in the wilderness. In modern Western Christian practise, Lent is the 40 days preceeding Easter.
- The age when life begins, according to the maxim "Life begins at forty".
- In the expression "Forty winks", meaning a short sleep.
- In years of marriage, the ruby wedding anniversary.
- The code for direct dial international phone calls to Romania.
- The designation of Interstate 40, a freeway that runs from California to North Carolina.
- The designation for US Highway 40, the 800-mile highway that runs from Baltimore, Maryland, to Vandalia, Illinois, being synonymous with the National Highway
- For the year AD 40.
