If less than half of the garden is planted with corn, then is it reasonable to estimate that 5/6 of the garden is planted with corn?

No because that means that you are saying that LESS than half should be without corn so if it's less it should be either 2/5 or 1/5. 

Among the choices the one that  best describes her prediction is "extrapolation, because a key that is 12 keys from middle C is outside the data set". 

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