Take another look at the original sentence:

Gustavo can tolerate any lecture, no matter how boring the instructor is, in his mind, Gustavo imagines that he is on board the starship Enterprise, blasting Romulans from his post on the bridge.

You chose to fix it the correct way:

Gustavo can tolerate any lecture, no matter how boring the instructor is; in his mind, Gustavo imagines that he is on board the starship Enterprise, blasting Romulans from his post on the bridge.

Gustavo can tolerate any lecture is the first main clause. In his mind, Gustavo imagines ... is the second main clause. You are always correct when you correctly use a semicolon between the two main clauses that create the comma splice.

Go to the next sentence.

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