R' Eliezer says: [Challah] may be taken from a tahor [dough] on behalf of a tamei [dough]. How? [If one has] a tahor dough and a tamei dough, he takes an amount sufficient for challah from the dough whose challah has not been taken, and places less than a beitzah [of dough] in the middle, so that he will be taking [challah] from something in proximity [to the doughs]. But the Sages forbid [it].

The measure of challah is one twenty-fourth [of the dough]. [For] one who makes a dough for himself, and [for] one who makes [a dough] for his son's feast, [the measure is] one twenty-fourth. [For] a baker who is making [bread] to sell at the market, and, likewise, [for] a woman who is making [bread] to sell at the market, [the measure is] one forty-eighth.  [If] her dough became tamei mistakenly or accidentally, [the measure is] one forty-eighth. [If] it became tamei intentionally, [the measure is] one twenty-fourth, so that the transgressor should not profit [from her transgression].