Explanation

You are told that the plumber is not professional, so you need a word for the first blank that is the opposite of “professional.” A layperson is a non-professional.

For the second blank, you need a word describing how a non-professional would work. Rote implies experience, and by numbers means “following simple instructions,” which is illogical because there’s no reason to believe the layperson plumber has any instructions. Guesswork means he’s just guessing what to do, which makes the most sense.

Finally, since the plumber successfully and quickly fixed everything, you need a word in the third blank that means “good.” Crack means “very good.” Extortionate means “overly expensive”; there’s no reason to believe someone is good just because they’re overpriced.

Layperson, guesswork, a crack.
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