A Montgomery County judge sentenced a Silver Spring man to 10 years in prison yesterday for enticing two girls, aged 11 and 13, to perform sexual acts in exchange for cocaine and marijuana.

Paul C. Black, 43, was convicted in November of two counts of second-degree sex offense and one count each of third-degree sex offense and distribution of cocaine. Black also was found guilty of possession of cocaine and marijuana.

Yesterday, Circuit Court Judge Jerry H. Hyatt imposed the maximum sentences recommended under the Maryland sentencing guidelines for the more serious offenses.

He sentenced Black to four concurrent 10-year terms on the sex-abuse and cocaine-distribution convictions.

Black, citing health problems, asked Hyatt to allow him to serve his sentence in the county jail. But Hyatt ordered that Black be incarcerated in the state prison system, where he will be eligible for parole after about three years.

Defense attorney Maureen Essex said yesterday the convictions and sentences will be appealed.

During a four-day trial, the two girls gave conflicting testimony. The 11-year-old girl testified that Black offered her and a friend crack cocaine on Oct. 9, 1989, while the girls were in the basement of Black's house in the 8700 block of Flower Avenue. The younger girl said Black told them, "You don't get this stuff for free."

However, the 13-year-old girl denied that she had performed sexual acts with Black. She also testified that Black and the girls smoked marijuana but not crack.

But prosecutor Frank Molony told the jury that police were able to substantiate key parts of the younger girl's statements during a December 1989 drug raid at Black's house.

Police confiscated cocaine and other drug paraphernalia during the raid, according to court testimony.

The mother of the 11-year-old girl reported the incident to authorities. Both girls, who formerly lived in Prince George's County, were being cared for by a boyfriend of the older girl's mother when the incident occurred.