Who knows one hundred fifty?
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150 years after the destruction of the second Beis Hamikdash (in the year 530 Seleucid Era = 3979 since Creation), Rav left Eretz Yisrael and returned to his native Babylonia, marking a new era in the study and dissemination of Torah there. (Iggeres R' Sherira Gaon)
According to R' Yehudah Halevi (Kuzari 3:67), this is also the year in which R' Yehudah Hanassi (Rabbeinu Hakadosh) completed the composition of the Mishnah.
Ways that the student was able to prove that a sheretz is tahor (eiruvin 13b)