The Jews entered the land of Eretz Israel from exile in Egypt in the year 2488 From Creation (F.C.). The Jews remained in Eretz Israel until the year 3338 F.C. when the Temple was destroyed and the Jews were sent into exile in Babylonia. The Jews returned from exile in Babylonia to build the Second Temple in the year 3408 F.C. (Rashi on Vaykra 26:35)
A. How long were the Jews in exile in Bablyonia?
3408 FC – 3338 FC = 70 years
B. How long was the period between the entry to Erez Israel from the exile in Egypt to the exile in Babylonia?
3408 FC – 2488 FC = 850 years
C. During the Yovel and Shmittah years the Jews were not allowed to work the land. The Shmittah is once ever seven years. After seven Shmittah cycles there is a Yovel year (i.e. once every fifty years). How many years did the land have to lay fallow during each 50 years?
7 shmittah + 1 Yovel = 8 years fallow.
D. Rashi said that the 70 years of exile in Bablyon were to make up for each of the years that the Jews failed to let the land lay fallow. If the Jews had failed to observe all of the Yovel and Shmittah years between the return from Egypt and the exile to Babylonia how many years would the Babylonian exhile have lasted?
850 / 50 x 8 = 136.
E. Rashi points out that since the exile was only 70 years because they only failed to keep Yovel and Shmittah for 436 years. How many full Schmittah/Yovel cycles take place in 436 years? How many years would be left over?
436 /50 = 8 cycles with 36 remainder
F. Of the years remaning how many schmitah cycles would there be? How many years would be left over?
36 / 7 = 5 years with 1 year left over. Rashi explains that there is one year left over because they started to ignore that shmittah cycle so it counted for an extra year of exile.