Peter Lowe, Apple’s director of marketing for applications and services, said Monday that more than 20 million songs have been purchased and downloaded from the iTunes Music Store since it launched in April. Lowe, who was a keynote speaker at the Music 2.0 event in Los Angeles, also said that 45 percent of songs downloaded through iTunes had been sold as part of a full album, rather than in single song form. Russ Crupnick, vice president of NPD Group, noted research that indicated iTunes users bought more music than did ordinary offline consumers. He said the average iTunes customer bought 49 songs online in the first four months of Apple’s store (about an album a month), compared with the average teenager’s purchase of a CD every two months.