As the Torah describes the preparations for the Exodus, the Torah writes that Bnei Yisrael lived in Egypt for 430 years (Shemot 12:40-41). Chazal ask that this seems to contradict the prophecy to Avraham that his descendants would be in a foreign land in which they would be enslaved and tortured for 400 years (Bereishit 15:13). They further contrasted it with a few hints that, from the time that Yaakov went down to Egypt, only 210 years passed until the Exodus, which is even less than half of the 430 that are mentioned.
Let us start by looking at the discrepancy between 400 and 430 years, both mentioned in explicit p’sukim. Mechilta D’Rashbi (Shemot 12:40) and Targum Yonatan (ad loc.), whose explanation is adopted by Rashi (Bereishit 15:13) and the Rashbam (Shemot, ibid.), explain that the 400 years are to be counted from the birth of Yitzchak, even though he personally did not go down to Egypt. The 430 years is from the time that the Brit Bein Habetarim was made with Avraham, when he was 70 years old. (That raises a question of timing, considering that Parashat Lech Lecha begins with Avraham being 75 years old (see Bereishit 12:4); we will discuss this in the soon-to-be-published Tzafnat Shmuel).
The Abarbanel cites an opinion that the exact number of years was that which is mentioned in Shemot – 430 years. The number 400 was an approximation. However, the Abarbanel continues with an explanation he prefers to the above. The decree was that which is found in Bereishit – 400 years. In practice, the exile was extended beyond its minimum because the Jewish People destroyed their proper path and allowed themselves to slip into acts of the abominations of the Land of Egypt. Therefore, 30 years were added on to their stay in Egypt. This is because all divine decrees are on the condition that they can be affected by the subsequent positive or negative behavior of the subject of the decree. Thus, 400 is what should have been under normal circumstances, and 430 was what was necessary as things transpired.
Based on this final approach, we can understand the Rabbinic tradition regarding the mistake of the Sons of Ephrayim. They tried to leave Egypt earlier than the rest of the nation and were killed on the "path of the Land of the Plishtim" (see Shemot 13:17 and Divrei Hayamim I,7:20-21). The gemara (Sanhedrin 92a) claims that the bones that Yechezkel brought back to life were those of the murdered Sons of Ephrayim, who left early. If the 400 years that Avraham was told had already passed, we can understand why they felt confident that they would be successful in leaving.
(According to the Psikta Zutrata, Shemot 12:40, that the actual time spent in Egypt was 210 years, neither 400 nor 430 years was meant in the literal manner we might expect, which lessens the apparent contradiction. We will continue with this next week.)