Genesis 15:13
13 And he said unto Abram, Know of a surety that thy seed shall be a stranger in a land that is not theirs, and shall serve them; and they shall afflict them four hundred years;
Acts 7:6
6 And God spake on this wise, That his seed should sojourn in a strange land; and that they should bring them into bondage, and entreat them evil four hundred years.
Exodus 12:40-41
40 Now the sojourning of the children of Israel, who dwelt in Egypt, was four hundred and thirty years.
41 And it came to pass at the end of the four hundred and thirty years, even the selfsame day it came to pass, that all the hosts of the Lord went out from the land of Egypt.
Galatians 3:16-17
16 Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
17 And this I say, that the covenant, that was confirmed before of God in Christ, the law, which was four hundred and thirty years after, cannot disannul, that it should make the promise of none effect.
Above in Genesis 15:13, we have God telling Abraham that his children will be afflicted for 400 years, which is referenced in Acts 7:6. However, we also have Exodus 12:40-41 telling us that the children of Israel were in Egypt for 430 years, which is referenced in Galatians 3:16-17.
So, was it 400 or 430 years?
There is some confusion regarding how many Christians attempt to answer this alleged contradiction, because many will try to start the timeline when Abraham moves to Egypt briefly in Genesis 12:10. However, he moves back to Canaan in Genesis 13:12, so this is not the prophesied period of time.
The time period begins when Abraham's grandson, Jacob (Israel), leaves the land of Canaan to settle in Egypt, when he is 130 years old (Genesis 47:6-9). At this time, one of Jacob's sons, Joseph, has become a ruler in Egypt, and Pharaoh gives the Israelites the land of Goshen in Egypt to reside in.
This is bolstered by the fact that Galatians 3:17 references the covenant being confirmed 430 years before the Exodus. This event takes place in Genesis 46:2-4, when Jacob is 130 years old.
So, how do we get 400 years of affliction? It has to be that the period of what God considered to be affliction began 30 years after they began to live in Egypt.
We know that Joseph was 39 when they moved to Egypt as a family, because he began to be a ruler in Egypt at about 30 years of age (Genesis 41:46), then there are 7 good years (Genesis 41:47), and then Joseph reveals himself to his brethren 2 years into famine that began afterward (Genesis 45:1-6). That would mean that the period of affliction began when Joseph was 69 years old, ostensibly when he was relegated to a lesser role in the kingdom of Egypt. Joseph dies at 110 years old (Genesis 50:26), 71 years into the children of Israel's residency in Egypt.
Then, we are told that after Joseph died, persecution really began to ramp up for the children of Israel in Egypt (Exodus 1:8-12), meaning this period of intense persecution lasted 359 years (430 - 71). It could also be that the 400 years is rounded from this total, but it's conceivable that a lesser persecution began 41 years prior to Joseph's death.
The Exodus took place exactly 430 years after the Israelites began to live in Egypt (Exodus 12:40-41). However, the period of affliction was either exactly 400 years, or rounded to 400 years from the period after Joseph died, which was 359 years before the Exodus.