And A Random Question

As I was going to sleep last night, my thoughts briefly turned to tithing. And I wondered why it is that we pay 10%. A lot of numbers of things in church have some sort of symbolic meaning: 3 and 12 are the most common, but I think 7 comes in there, too. But why ten? The only reason I can think for that particular number is that it makes figuring out how much to pay pretty easy using our numbering system.