Here are two very good posts regarding the Gemora on 20b which is extremely technical. Rabbi Avi Lebovitz has a shiur here that you can download and listen to. The Kollel Iyun Hadaf under the leadership of Rabbi Mordechai Kornfeld does a fabulous job on every daf and every aspect of the daf. Please click here where they discuss extensively Rashi's opinion on the sugya which is extremely difficult to grasp.
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