There is a loophole in this question: "every teacher in 24 classes in the whole school should have a tape recorder." The 24 classes in the whole school are not the key point, but each teacher should be equipped with a tape recorder, so the key point is how many teachers there are in the whole school and how many tape recorders are needed, which is not given in the title.

Assuming that only 24 tape recorders are bought, the method of 20% discount is more cost-effective: assuming that 100 yuan/set, if you buy 24 sets, the first set needs 1920 yuan and the second set needs 1 set, so you only need to buy 20 sets to get 4 sets, so you need 2,000 yuan.