An 18-year-old youth pleaded guilty yesterday do murdering a Gaithersburg man he said he had never met in return for $50 and the promise of an additional $70 and a stereo set on completion of the crime.

Immediately after Andrew Leon Hart entered his plea in Montgomery County Circuit Court, prosecutors introduced a statement in which Hart told of meeting a man he knew only as "Vic" and of agreeing to shoot the boyfriend of Vic's wife. In the statement, Hart told of riding his bicycle from his home in Emory Grove to shoot at a man through the window of an apartment in Gaithersburg. Police said Hart didn't know the victim's name.

The gunfire last July 12 fatally wounded Michael James Alexander, 21. Alexander died the following month, police said. Last September, Hart and Gaithersburg businessman Victor L. Saadeh were charged in the slaying.

Hart, who had been charged with first-degree murder, was allowed to plead guilty to second-degree murder in return for agreeing to testify agains Saadeh.Hart could be sentenced to up to 30 years in prison. Sentencing is scheduled for April 23, the same day Saadeh is to be tried on a charge of conspiring to commit murder.

In his statement to police, Hart said a friend named Earl took him last July to meet "Vic" at a television repair shop and that Hart agreed to kill a man for $100.

That same night, according to the statement, "Vic" drove Hart to a Gaithersburg apartment building and pointed out the intended victim through a window, saying "he wanted the guy killed... he wanted it done as soon as possible." Hart said the man gave him $50 with the promise of $50 more when the job was done.

At the time, Hart was 17 and had dropped out of Magruder High School to help support an infant son living with his 15-year-old girlfriend, according to Hart's older brother, George Kevin Hart.

Andrew Hart was on probation after being judged delinquent in juvenile court for disorderly conduct and the theft of his uncle's car, according to court records.

At one point in the week before the shooting, "Vic" offered Hart a stereo set once the job was done, and said that he would pay Hart $20 for "taking off work to do it," according to Hart's statement. Hart told police he called in sick that night to his part-time job as a maintenance man, and went to the apartment, but found no one home.

On July 12, Hart bicycled to the building again, walked along the rear until he reached the apartment's living room window, according to the statement. When a drawn window shade fluttered open, Hart saw a man seated on a couch, according to the statement, and, as the shade flopped back Hart placed his gun against the outside window and fired.

Alexander, who shared the apartment with Victor Saadeh's wife, Darla, according to police, was wounded in the abdomen.

Hart threw his gun in the woods, bicycled home and went to bed, according to the statement.

Hart, who is black, has been portrayed in earlier court proceedings as a youth struggling to fit in a predominantly white, middle-class area, and whose earliest recollections of school involve children teasing him about wearing handed-down clothes.

One of nine children, Hart was singled out for beatings by his father, who died when Hart was 14, according to a court-ordered report by a worker for the Department of Juvenile Services. He was beaten "to the point that his nose would bleed, his mouth would bleed, and blood would come out of his ears," according to the juvenile worker's report.

Police testified yesterday that Alexander had met Darla Saadeh while both were in the detoxification unit at Montgomery General Hospital and "the two fell in love."

Alexander's sister. Lila Thornley, who attended yesterday's hearing, said her brother was "just straightening out" after a long-term drinking problem when he met Darla Saadeh in the hospital.

At the time of his arrest last September, Victor Saadeh, 32, was the owner of Aero TV, a repair shop at 12-D East Diamond Ave., Gaithersburg.