Biography

78 (seventy-eight) is the natural number following 77 and preceding 79.

In mathematics

78 is:

the 4th discrete tri-prime; or also termed Sphenic number, and the 4th of the form (2.3.r).^[1]
an abundant number with an aliquot sum of 33; within an aliquot sequence of nine composite numbers (78, 90,144,259,45,33,15,9,4,3,1,0) to the Prime in the 3-aliquot tree.
a semiperfect number, as a multiple of a perfect number.
the 12th triangular number.
a palindromic number in bases 5 (303₅), 7 (141₇), 12 (66₁₂), 25 (33₂₅), and 38 (22₃₈).
a Harshad number in bases 3, 4, 5, 6, 7, 13 and 14.
an Erdős–Woods number, since it is possible to find sequences of 78 consecutive integers such that each inner member shares a factor with either the first or the last member.^[2]
the dimension of the exceptional Lie group E₆ and several related objects.
the smallest number that can be expressed as the sum of four distinct nonzero squares in more than one way: $8^{2}+3^{2}+2^{2}+1^{2}$ , $7^{2}+4^{2}+3^{2}+2^{2}$ or $6^{2}+5^{2}+4^{2}+1^{2}$ (see image).^[3]^[4]

77 and 78 form a Ruth–Aaron pair.

78 is also:

In reference to gramophone records, 78 refers those meant to be spun at 78 revolutions per minute. Compare: LP, 33+1⁄3 and 45 rpm. 33 + 45 = 78
A typical tarot deck containing the 21 trump cards, the Fool and the 56 suit cards make up 78 cards
The Rule of 78s is a method of yearly interest calculation
The number used by Martin Truex Jr. and Furniture Row Racing to win the 2017 Monster Energy NASCAR Cup Series championship and 2016 Coca-Cola 600. The team and driver Regan Smith also won the 2011 Showtime Southern 500 with 78. The number is now used by owner-driver B.J. McLeod for Live Fast Motorsports.