How Long Were They in Egypt?

How Long Were They in Egypt?
In Genesis 15:13, it states that they were oppressed in Egypt for four hundred years; however, in Galatians 3:17 and Exodus 12:40, it says they lived in Egypt for four hundred thirty years.

To answer this supposed contradiction, there are many explanations. The ESV Study Bible provides a few explanations. Paul, the author of Galatians, may be referring to the Septuagint translation of Exodus 12:40 which is slightly different from the Hebrew manuscripts. Instead of saying that they lived in Egypt for 400 years, the LXX (or Septuagint) says that they lived in Egypt and Canaan for 430 years (Crossway 2250). 

It also may be that 400 "[is] understood as a round figure" (Crossway 77). The exact number is 430 and 400 is rounded (Cambridge; Ellicott; Keil).

References:

Ellicott's Bible Commentary, Volume 1. N.p., Delmarva Publications, Inc., 2015.

ESV Study Bible. United States, Crossway, 2008. 

Keil, C. F., & Delitzsch, F. (1920). Keil and Delitzsch Commentary on the Old Testament. Library of the Union Theological Seminary. 

New American Standard Bible®, Copyright © 1960, 1971, 1977, 1995 by The Lockman Foundation. 

The Cambridge Bible for Schools and Colleges. United Kingdom, The University Press, 1905.