Thank you for the question, Russell.
The letter grades in your example DO NOT need English Letter indicators when they are immediately followed by a symbol of operation; the letter grades in your example DO need English Letter indicators when they are "single letters" (without a sign of operation). [See section 25 of the Nemeth Code.]
For each letter-operation-sign-apostrophe-s sequence, the punctuation indicator should precede the apostrophe and the sequence should be punctuated mathematically; for each letter-apostrophe-s sequence, no mathematical punctuation is necessary. See section 27.e of the Nemeth Code for some relevant examples. The attached image contains a suggested transcription for the portion of the text you asked about.
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