A MAN was jailed for 20 months for living on the earnings of four under-age prostitutes. Li Man-fai, 24, who absconded in 1988, pleaded guilty in the District Court to three counts of living on the earnings of prostitutes. He admitted to a similar charge on Tuesday. The court heard that between October 1987 and July 1988, Li persuaded four girls, aged from 14 to 16, to work as prostitutes in return for food and lodgings. Although clients were charged between $180 and $200, no money was given to the girls, the court was told. One girl, a 15-year-old virgin, had sex with a customer after Li told her she could earn $6,000. ''He kept them dry and he kept them fat no more than any man would do for his horse and dog,'' Judge Hartman said.