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Now for a few important points. First, "memory" almost always refers to "RAM" or "random access memory." This is the solid-state hardware that the computer uses to handle information quickly-it is "volatile" (it goes away when power is removed), and is typically limited in capacity to around 8GB.
A "hard drive" is NONvolatile (it retains its contents even after power is removed), and can be enormous, with current drives offering up to 2 TERRAbytes of storage.
Your MacBook problem appears to be one of HARD DRIVE storage. If you started out with 111GB, you can expect around 10-15GB of that to be eaten up by the OS. Your roughly 21GB of music would put used space at a maximum of about 36GB. But what apps do you have installed? That can be a big thing. Apps take up disk space, sometimes a whole lot of it. Your documents and files could be large as well. And OS X creates logs for a number of operations as well, and these logs may not be purged when no longer needed. There's a lot that could be eating up your hard drive's space. Take a look at things other than music on your drive-you may have tons of gunk you don't need to keep.