Take another look at the original sentence:

In Dr. Ribley's calculus class, Yi-Ping tapes every lecture and carefully rewrites her notes, she is planning to sell these guides to desperate freshmen next semester.

You wanted to fix it this way:

In Dr. Ribley's calculus class, Yi-Ping tapes every lecture and carefully rewrites her notes she is planning to sell these guides to desperate freshmen next semester.

To delete the comma between notes and she would cause an equally bad problem, a fused sentence. A fused sentence occurs when you have two main clauses joined with no punctuation whatsoever. Yi-Ping tapes every lecture is one main clause. She is planning to sell these guides to desperate freshmen next semester is the second main clause. The spot between notes and she needs a stronger break than blank space.

You might want to consult the rules for fixing comma splices and fused sentences.

Go back to the sentence to try again.

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