Rashi and Targum Yonasan: They were arguing that, since their father left no sons, either they should stand in place of sons [and inherit], or their mother should do Yibum with their father's brother
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(and inherit both their father's portion and that of his deceased brother ? Targum Yonasan).
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Because they were Chochmaniyos, and approached Moshe as he was learning the Dinim of Yibum. See Torah Temimah on Pasuk 5, citing Bava Basra, 119b and note 10. See also Oznayim la'Torah on Pasuk 1, DH 'va'Tikravnah ... va'Ta'amodnah'.
What prmpted the B'nos Tz'lofchad to state their claim now?
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Targum Yonasan and Sifri: Because they heard that only the males would inherit Eretz Yisrael (so they discussed among themselves what to do,
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and came to the conclusion that, if people tend to favor males over females, Hashem has equal compassion on both, so they took their case to Moshe to place before Hashem - Sifri).
Bearing in mind that the Manna was about to cease, and they had no father or brother to sustain them. See Oznayim la'Torah on Pasuk 3, DH 'Avinu Meis ba'Midbar', who discusses their problem at length.
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Why did they add that he had no sons?
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Rashi: This implies that had he had a son, they would not have claimed - to teach us that they were Chochmaniyos.
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Sifri: Having already stated ""u'Vanim Lo Hayu lo",
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they added "ki Ein lo Vein", to teach us that, even if he would have had the son of daughter, they would not have claimed - because they were Chachmaniyos and Darshaniyos.
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