640

640 = 2⁷ x 5.

640 has a representation as a sum of two squares: 640 = 8² + 24².

100n² - 640n + 2(640) - 100 + 7 gives distinct primes for n = 0 to 24.

640 is 22000 in base 4 and 1200 in base 8. It is 454 in base 12.

640 is the number of acres in a square mile.

Source: Prime Curios!