[math-fun] What use is algebra, anyway?

As I waited at the checkout of our local wholefood shop, a lady who I know to be both literate and equipped with a technical background --- besides a number of other attributes which, though perhaps not immediately relevant to the discussion, hardly serve to encourage coherent impromptu cogitation on the part of an elderly academic --- conveyed to me an enquiry posed by her teenage daughter, along the lines of: "You're a mathematician" (always an ominous opener, I have observed) "--- what's the use of algebra?" I think I did eventually manage to marshall a reasonably convincing reply. But I wondered what would other people here have told her? Fred Lunnon