78

78 is a natural number that follows 77 and precedes 79. It is an even composite number with prime factorization 2 × 3 × 13. It is also a triangular number, equal to the sum of the integers from 1 to 12 (12 × 13 / 2 = 78).

Its divisors are 1, 2, 3, 6, 13, 26, 39, and 78, giving eight divisors in total. The sum of its proper divisors is 90, which makes 78 an abundant number. In various numeral systems, 78 is represented as binary 1001110, hexadecimal 0x4E, and octal 116; in base 3 it is 2220_3, in base 5 it is 303_5, and in base 7 it is 141_7. In Roman numerals, 78 is written as LXXVIII.

78 has several notable mathematical associations. It equals C(13,2), the number of unordered pairs formed from a 13-element set. It is also the 12th triangular number, tying it to a classic sequence of figurate numbers.

In science and culture, 78 has other associations. It is the atomic number of platinum, a dense noble metal used in catalytic converters, jewelry, and various industrial applications. The term 78 rpm refers to an early standard for phonograph records, a format widely used during the first half of the 20th century before the advent of vinyl LPs.