Demai, Chapter 5, Mishnah 1. This mishnah deals with someone who buys bread from a baker who is an am ha'aretz, and wants to make two separations at once: terumas maaser from the demai - which is ultimately the gift he is obligated to give to the kohen - and also challah, which is likewise given to the kohen. How does he accomplish it all in one stroke?
The amounts he must separate:
Terumas maaser - one hundredth: Maaser is one tenth, and terumas maaser is one tenth of the maaser, so its measure comes out to one hundredth of the total quantity.
Challah - one forty-eighth: That is the measure in this case.
The wording of the separation:
He takes that one hundredth portion and also the portion that is one forty-eighth, and says as follows:
"That one hundredth of the total quantity of all this bread shall be considered maaser" - this is not yet terumas maaser, since terumas maaser is separated only after maaser exists, and the other nine parts of the maaser are lying right next to it.
"That very first portion, the one hundredth that I made maaser, shall be considered terumas maaser for the rest of the maaser".
"And the remainder that I set aside at the outset, the one forty-eighth, shall be challah".
What he must give and what he is exempt from:
The halachah is that with demai he is not obligated to give the maaser to a Levi, because we say to the Levi, "Bring proof that tithes were never separated from this." But the terumas maaser he is obligated to give to a kohen, and likewise the challah that he separated.
Maaser sheni:
He is still responsible to take care of maaser sheni. He therefore designates that the maaser sheni be on the northern side of the loaf or on its southern side, and it is transferred and redeemed onto the money he set aside for redeeming maaser sheni.
In summary: This mishnah taught how someone who buys bread from a baker who is an am ha'aretz arranges all of his separations at once: one hundredth becomes maaser and then terumas maaser for the rest of the maaser, one forty-eighth becomes challah - and both are given to the kohen - and the maaser sheni is designated on one side of the loaf and redeemed onto the money. Once all of this has been done, he may go ahead and eat the rest of the loaf of bread.