听力原文：W: I'm exhausted. It took me about three hours just to buy a few things. M: What did you buy? W: I bought a dozen cassettes. M: How much did they cost you? W: I bought them on sale. They were $ 20 a dozen. M: Including tax? W: No. I paid another $ 0.80 in tax. M: So the tapes cost you $ 20.80. W: Right. Only I had to take the subway to and from the store, so that cost me another $ 1.20. M: So the cassettes really cost you $ 22 a dozen. W: Right. Altogether I spent $ 22. What did you buy today? M: A half-dozen cassettes. W: You're kidding! M: No. I had the same idea you had. I saw an ad in this morning's paper so I went down to buy them. W: What did they cost you? M: I bought them for $ 2 a piece, including tax. W: So you spent $ 12 on six tapes. M: Yes. And I bought them in the neighborhood so I spent only a few minutes shopping and I didn't spend any more money on transportation. W: But my tapes still cost a total of $ 2 less than yours. M: Sure, only you spent $ 22 on a dozen 60-minute tapes and I bought a half-dozen 90 minute tapes for $ 12. W: Oh. It sounds like you really did get a bargain. But let's check out the quality of our tapes and find out if either of us really got a good deal. M: We should have thought of that before we bought them. W: True. But we didn't. (20)