Why did Yosef not accept their answer?
Rashi: After ascertaining that if they found their lost brother, they would pay any ransom that was asked of them, Yosef asked them what they would do if his master would refuse to sell him, to which they replied that they were willing to kill or to be killed on his behalf. That was when he reiterated his accusation. 1
Malbim: Until now, I thought that your father had [about] 100 sons. 2 It is reasonable that he would send 10 to buy food. Now that you say that only one son remains there, surely he would not send 10. You must be messengers (Meraglim) from 10 different Shevatim!
Seforno: He claimed that the 'missing' brother, whose location they refused to divulge, had gone back to report to their father what they had seen, or that they had decided to spy out the land.
Ha'amek Davar: It is unreasonable that you are brothers, and your father would leave [only 3 ] one son at home. You did not prove that you are Kenim (have an inheritance, wives and children). One brother is not - presumably, he was caught spying! The only way to test you is through the small brother.
Rashi: Adding that he had divined by means of his goblet that two of them had wiped out Shechem.
Why did he assume so? Perhaps Yosef gauged that one man with a donkey can buy for 10 men. (Really, there were 66 (or 78 - refer to 46:26:2 in Yaakov's household, including all his descendants! Yosef did not ask how many are in the household! Perhaps he held that one man can buy for 10 [small] families. However, a donkey carries only 15 Se'ah (Rashi to Bava Metzia 80a). The poorest Yisrael must supply his wife with a third of a Se'ah of wheat each week (Kesuvos 64b, 65b). Even if there are only four people per family, 10 families would finish a donkey's load in just over a week. Some parts of Eretz Kena'an are several weeks journey from Egypt! (PF)
It seems that this is the intent, even though this word is not in the text. (PF)
QUESTIONS ON RASHI
Rashi writes: "'Hu Asher Dibarti' - That which I have said, that you are spies, is correct (Hu ha'Emes v'ha'Nachon)." What does Rashi mean?
Gur Aryeh: It is preferable to translate simply, that Yosef responded from within the brothers' own words, 'But that (Hu) is was I said - that you are spies!' But the brothers had not said any such thing! 1 Rashi therefore turns the phrase around and puts "Hu" at the end.
Gur Aryeh: In Rashi's second explanation, from the Midrash, the brothers in fact declare that they are ready to kill in order to free the missing Yosef -- to which the viceroy responds, "But that (Hu) is was I said...."
