What follows are a bunch of calculations. Since the magicians called a single plague the finger of God, and at the Reed Sea there is reference to the great hand, we multiply by 5.
To what end? To praise Hashem for providing so many miracles, and so to demonstrate that there were so many miracles. Also, because כל המרבה לספר ביציאת מצרים הרי זה משובח, and this is a form of being marbeh.
Thus, a calculation of 10 and 10 x 5.
Don't spend too much time on these derashot. Say it in English, in summary, of how one derasha differs from the next and what the conclusion is.
Rabbi Eliezer brings a prooftext in which each plague in Egypt has four plagues, or four aspects:
We should not look for deep meanings of these derashot, and figure why each said what he said. These derashot were said in chronological order, in the order given here. Had R' Akiva spoken second, then he would have given R' Eliezer's derasha.
Why? The point was Rabbinic creativity and one-upmanship in creating derashot to praise Hashem. And this was their avodah, perhaps even on the seder night.