Mishnah Taharos 1:5: Food which has become impure from an origin of impurity [and has thus attained a first degree level of impurity], and [food] which has become impure from a generated impurity [such as something of a first degree level, causing it, in turn, to attain a second degree level of impurity], they are added together to impart impurity [at the level] of the less stringent of the two. How so? The equivalent [in volume] of half an egg of food of a first degree [level of impurity] and the equivalent [in volume] of half an egg of a second degree [level of impurity] that one mixed with each other, [the mixture] is of a second degree. The equivalent of half an egg of a second degree and the equivalent of half an egg of a third degree that one mixed with each other, [the mixture] is of a third degree. The equivalent of a full egg of a first degree and the equivalent of a full egg of a second degree that one mixed with each other, [the mixture] is of a first degree; but if one [then] divided them [into two], this one is of a second degree and this one is of a second degree. If one [of these two] fell on its own and [the other] one [also fell] on its own onto a loaf of terumah, they invalidate it [by giving it a third degree level of impurity]. If both fell at once they render it to be of a second degree [impurity, since it came into contact with a full egg's measure of food of a first degree impurity].

הָאֹכֶל שֶׁנִּטְמָא בְאַב הַטֻּמְאָה וְשֶׁנִּטְמָא בִוְלַד הַטֻּמְאָה, מִצְטָרְפִין זֶה עִם זֶה לְטַמֵּא כַקַּל שֶׁבִּשְׁנֵיהֶן. כֵּיצַד. כַּחֲצִי בֵיצָה אֹכֶל רִאשׁוֹן וְכַחֲצִי בֵיצָה אֹכֶל שֵׁנִי שֶׁבְּלָלָן זֶה בָזֶה, שֵׁנִי. כַּחֲצִי בֵיצָה אֹכֶל שֵׁנִי וְכַחֲצִי בֵיצָה אֹכֶל שְׁלִישִׁי שֶׁבְּלָלָן זֶה בָזֶה, שְׁלִישִׁי. כַּבֵּיצָה אֹכֶל רִאשׁוֹן וְכַבֵּיצָה אֹכֶל שֵׁנִי שֶׁבְּלָלָן זֶה בָזֶה, רִאשׁוֹן. חִלְּקָן, זֶה שֵׁנִי וְזֶה שֵׁנִי. נָפַל זֶה לְעַצְמוֹ וְזֶה לְעַצְמוֹ עַל כִּכָּר שֶׁל תְּרוּמָה, פְּסָלוּהוּ. נָפְלוּ שְׁנֵיהֶן כְּאַחַת, עֲשָׂאוּהוּ שֵׁנִי:

Mishnah Taharos 1:6: The equivalent of an egg [in volume] of a second degree food and the equivalent of an egg [in volume] of third degree food that one mixed with one another, [the mixture] is of a second degree; if one divided them [into two], this one is of a third degree and this one is of a third degree. If one [of these two] fell on its own, and [the other] one [also fell] on its own onto a loaf a terumah, it has not invalidated it [since terumah is not made impure by something of third degree impurity]. If both fell at once they render it to be of a third degree [impurity]. The equivalent of an egg [in volume] of first degree food and the equivalent of an egg [in volume] of third degree food that one mixed with each other, [the mixture] is of a first degree; if one divided them [into two], this one is of a second degree and this one is of a second degree, since even that of a third degree, when it touched that of a first degree, it became of a second degree. The equivalent of two eggs [in volume] of first degree food and the equivalent of two eggs [in volume] of a second degree food that one mixed with each other, [the mixture] is of a first degree; if one divided them [into two], this one is of a first degree and this one is of a first degree; if [one divided them] into three or four, they are [all] of a second degree. The equivalent of two eggs [in volume] of second degree food and the equivalent of two eggs [in volume] of third degree that one mixed with each other, [the mixture] is of a second degree; if one divided them [into two], this one is of a second degree and this one is of a second degree; if [one divided them] into three or four, they are [all] of a third degree.

כַּבֵּיצָה אֹכֶל שֵׁנִי וְכַבֵּיצָה אֹכֶל שְׁלִישִׁי שֶׁבְּלָלָן זֶה בָזֶה, שֵׁנִי. חִלְּקָן, זֶה שְׁלִישִׁי וְזֶה שְׁלִישִׁי. נָפַל זֶה לְעַצְמוֹ וְזֶה לְעַצְמוֹ עַל כִּכָּר שֶׁל תְּרוּמָה, לֹא פְסָלוּהוּ. נָפְלוּ שְׁנֵיהֶן כְּאַחַת, עֲשָׂאוּהוּ שְׁלִישִׁי. כַּבֵּיצָה אֹכֶל רִאשׁוֹן וְכַבֵּיצָה אֹכֶל שְׁלִישִׁי שֶׁבְּלָלָן זֶה בָזֶה, רִאשׁוֹן. חִלְּקָן, זֶה שֵׁנִי וְזֶה שֵׁנִי, שֶׁאַף הַשְּׁלִישִׁי שֶׁנָּגַע בָּרִאשׁוֹן נַעֲשָׂה שֵׁנִי. כִּשְׁתֵּי בֵיצִים אֹכֶל רִאשׁוֹן כִּשְׁתֵּי בֵיצִים אֹכֶל שֵׁנִי שֶׁבְּלָלָן זֶה בָזֶה, רִאשׁוֹן. חִלְּקָן, זֶה רִאשׁוֹן וְזֶה רִאשׁוֹן. לִשְׁלשָׁה אוֹ לְאַרְבָּעָה, הֲרֵי אֵלּוּ שֵׁנִי. כִּשְׁתֵּי בֵיצִים אֹכֶל שֵׁנִי וְכִשְׁתֵּי בֵיצִים אֹכֶל שְׁלִישִׁי שֶׁבְּלָלָן זֶה בָזֶה, שֵׁנִי. חִלְּקָן, זֶה שֵׁנִי וְזֶה שֵׁנִי. לִשְׁלשָׁה אוֹ לְאַרְבָּעָה, הֲרֵי אֵלּוּ שְׁלִישִׁי: