Both comma uses are incorrect. They should be: I explained the questions in the exercise sheet during the first half of the class, and then students... The new arrangement was that during the first 50 minutes students work on their exercises and I answer all questions one by one at the same time; whereas, during the last 10 minutes I....
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uktousin my limited understanding your use of comma is correct
however, in my understanding (which may be wrong), you are better to write "to toss" rather than tossing
thanks
I agree... you seem to have a good understanding of how to use commas in this sentence.
However, it's incorrect to use "to toss" instead of "tossing" in your sam