God told Avraham (Abraham) that his children would be living in “lands not their own” and would be enslaved for 400 years, Yet the Torah states that the Israelites were in Egypt for 210 years.  So we say God “calculated the end” (chisheiv et ha'keitz) of the enslavement: He started the counting from the birth of Avraham's son, Yitzchak (Isaac), until the Exodus (a total of 400 years), as follows:
 
Time from Birth of Avraham's Son to Going Down to Egypt

• Yaakov (Jacob) was born when Yitzchak was 60 years old.
• Yaakov told Par'o (Pharoah) (when he and the rest of his family entered Egypt) that he was 130 years old.
• 60 + 130= 190 years before going down to Egypt

 
Time in Egypt
210 years in Egypt
 
Time from Birth of Avraham's Son to Exodus
  190 Before going down to Egypt
+ 210 In Egypt
= 400 years from Yitzchak's birth to the Exodus.