Burton G. Tremaine, 89, who with his late wife, Emily, assembled one of the finest collections of 20th-Century art in the country. Tremaine was the son of a co-founder of Nela Park, the company that later became the lamp department of General Electric. By the late 1980s the Tremaines had assembled a collection of more than 400 works by various European and American artists. In 1980 the Tremaines sold “Three Flags,” by Jasper Johns, to the Whitney Museum for $1 million, believed at the time to be the highest price ever paid for the work of a living artist. In 1988, the year after Emily Tremaine died, Christie’s auctioned off 32 pieces from their collection for $25 million. In Rancho Mirage on Saturday.