100

100 = 2² x 5².

100 has a representation as a sum of two squares: 100 = 6² + 8².

100 is the sum of the first four cubes: 100 = 1³ + 2³ + 3³ + 4³. It is the smallest square that is the sum of consecutive cubes.

100 is the sum of the 9th and 10th triangular numbers: 100 = 45 + 55.

100 is the sum of the first 10 odd numbers: 100 = 1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 17 + 19.

The first three primes add to 10, and the first 3² primes add to 10² = 100.

The Romans wrote 100 as C. This is an abbreviation for the Latin word centum. A cent is a hundredth of a dollar, and a centime was a hundredth of a franc.

Source: Prime Curios!