Seventy-eight

Seventy-eight (78, LXXVIII) is the natural number following seventy-seven and followed by seventy-nine. It is a triangular number as well as a composite number, with its divisors being 1, 2, 3, 6, 13, 26, 39 and 78, and its factorization:

makes it a sphenic number.

Karl A. Dahlke proved in 1989 that 78 heptominoss of this shape ( image:heptomino-78.png), at a minimum, must be used in order to fill a rectangle.

78 is also:

The atomic number of platinum.
The designation of Interstate 78, a freeway that runs from Pennsylvania to New York.
In reference to analogue disc records, 78 refers those meant to be spun at 78 revolutions per minute. Compare: LP, 33 and 45.
A standard Tarot deck containing the 22 cards of the major arcana (including the fool) and the 56 cards of the minor arcana make up 78 cards.
The total number of gifts in the song The Twelve Days of Christmas (since 78 is the 12th triangular number).