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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 ( ), at a minimum, must be used in order to fill a rectangle.

78 is also:

See also: seventy