"seven times ten; the number which is one more than sixty-nine; a symbol representing this number;" Old English (hund)seofontig, from seofon (see seven) + -tig (see -ty (1)). Similar formation in Old Saxon sibuntig, Old Frisian soventich, Middle Dutch seventich, Old High German sibunzug, Old Norse sjautugr.
Seventy-eight (78) "gramophone record played at a speed of seventy-eight revolutions per minute," which was the standard until the introduction of long-play (see LP) in 1948; the use of seventy-eight to distinguish the old discs is by 1951.