Apple CEO Tim Cook just announced that Apple sold six million copies of Lion, the company’s latest operating system. Apple released Lion on July 20, adding new iOS-like features to the OS operating system and improving system performance.

To put this into perspective, Apple sold that number over three months while Microsoft sold 240 million copies of Windows 7 in 2010, or about 20 million a month. However, Cook reported that the OS sold 80% better than the previous release, Snow Leopard.

Gartner estimated that 4.5 percent of new PCs would ship with OS X in 2011 and expects 5.2 percent to ship with OS X in 2015. Cook said that the “the MacBook Pro and iMac are the number one best selling notebook and desktop in the US. This momentum is not just limited to a mode. The Mac outgrew the PC market by almost six times.”

“It took Windows 7 20 weeks to reach 10% of installed base. Lion did this in two weeks,” said Cook.