How will broadcast behave with a certain capacity?
I assume you want to broadcast from A.

The fastest is probably to broadcast at 80 Mbps and the end of the broadcast is then at 200 seconds (3.33 minutes) for both B and C.

Broadcasting at 100 Mbps will prevent C for receiving your file, since your switch can't store the exceeding 400 MB.

But I don't understand where do your constraints come from? Would it be possible to make C and B communicate? Also I don't quite understand how you did your calculations.