We can establish the first two cases of the Mishnah with two cows, and the last case with three cows.

(The first cases): Reuven borrowed two cows for half the day and rented them for half the day, or he borrowed them for today and rented them for tomorrow. Shimon claims that both died at the time they were borrowed. Reuven admits that one died when it was borrowed, and he is unsure about the other.

Since Reuven must swear and he cannot, he must pay.

(The last case): Reuven borrowed two cows and rented one. Two of them died. Shimon claims that they were the two borrowed ones. Reuven is unsure; perhaps only one was borrowed and the other was rented.

Since Reuven must swear and he cannot, he must pay.

Rami bar Chama holds that Shomrim must swear only if they partially admit to a claim. He can establish the first two cases of the Mishnah with three cows, and the last case with four.

(The first cases): Shimon claims that Reuven borrowed three cows for half the day and rented them for half the day, (or he borrowed them for today and rented them for tomorrow), and that all died at the time they were borrowed. Reuven admits that he received two cows from Shimon, and that one died while borrowed. He is unsure when the other died, and he denies that he took a third;

Since Reuven must swear and he cannot, he must pay.

(The last case): Shimon claims that Reuven borrowed three cows and rented one, and that the three borrowed cows died. Reuven admits that he borrowed two cows and rented one, and that one borrowed cow died. He is unsure if the other dead cow was borrowed or rented, and he denies that he took a fourth.

Since Reuven must swear and he cannot, he must pay.

(Mishnah): If Shimon claims that the borrowed cow died and Reuven says that the rented cow died, Reuven swears and he is exempt.

Question: Why must Reuven swear? He did not admit to the claim, rather to something else!

Answer (Ula): Since even according to Reuven's claim, he must swear that it died normally, Shimon can demand that he swear through Gilgul that it was rented.

(Mishnah): If both are unsure, Reuven pays half.

The Mishnah is Sumchus, who says that when we are in doubt, we divide the money.

Question (R. Aba bar Mamal): If he borrowed a cow b'Ba'alim and before returning it he rented it not b'Ba'alim (and it was lost or stolen), what is the law?

If the rental a separate matter, it is not b'Ba'alim, so he is liable;

Or, perhaps rental is a continuation of borrowing, for a borrower has the liability of a renter (for theft or loss) and more! (If so, also it is considered to be b'Ba'alim.)

Question (R. Aba bar Mamal): If you will say that rental is a continuation of borrowing, if he rented b'Ba'alim and before returning it he borrowed it not b'Ba'alim, what is the law?

Surely, borrowing (has more liability, so it) is not a continuation of renting, so he is liable;

Or, since it is a continuation regarding theft or loss, perhaps it a continuation for everything?

Question (R. Aba bar Mamal): If you will say that a partial continuation is not like a total continuation, if he borrowed b'Ba'alim, rented not b'Ba'alim, and borrowed it again, what is the law?

Does it revert to the original borrowing, which was b'Ba'alim?

Or, since he interrupted with a rental, it is a new borrowing not b'Ba'alim?

Question (R. Aba bar Mamal): If he rented b'Ba'alim, borrowed not b'Ba'alim, and rented it again, what is the law?

Does it revert to the original rental, which was b'Ba'alim?

Or, since he interrupted with borrowing, it is a new rental not b'Ba'alim?

These questions are not resolved. (Rambam - whenever the Gemara says 'if you will say', it is according to the Halachah.)

(Mishnah): If Reuven lent to Shimon a cow, he sent it with his own son, slave or Shali'ach, or with Shimon's son, slave or Shali'ach, and it died, Shimon is exempt;

If Shimon asked Reuven to send it with one of them, or if Reuven told Shimon that he will send it with one of them and Shimon agreed, Shimon is liable.

The same applies to returning it.