Answer: Well, it's hard to say what the "normal" rate of growth is, since so many factors (light, temperature, etc.) affect growth. That's why scientists use "control" plants--plants that receive the same growing conditions as the test plants, minus the factor being tested. Then they repeat the experiments over and over, with lots of plants, to see if they get the same results each time.
As far as how much the addition of fertilizer increases growth--that, too, depends on a lot of things. It depends on the type of soil used (and the amount of nutrients available to the "control" plants vs. the "test" plants), the dilution rate, etc.
It sounds like you have two experiments to run here--one comparing the growth of plants exposed to music with those not exposed to music. And one comparing the growth of plants fertilized with MiracleGro versus those not fertilized. Don't try to test both things on one set of plants--it's best to have just one "variable". Your teacher should be able to help you set up a good experiment. Good luck!
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