At every table there is a really good working computer.
Rationale:
good is an adjective
adjectives are modified by adverbs
really is an adverb (real is an adjective)
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It would be possible to say the following, but I don't think it's what you mean:
At every table there is a real, good, working computer.
The above means something like:
At every table there is a good working computer that is real (not imaginary or fake).
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Finally, you sometimes hear even native speakers of English say that something is "real good", but this is not standard (or "correct") English and I would not recommend its use in an exam. Adjectives are modified by adverbs.