This is my reply to Tag-MehirTzedek
who wrote:
I do not think the 1/5 th only leaving thus 80% dying is a Pshat statement.
The Bible tells us that after the first seven years of marriage, Yaakov (Jacob) had 4 wives that produced 11 sons and 1 daughter. His final son Binyamin was born several years later.
Approximately during the first 91 years in Egypt, give or take a year, the Israelites had it good in Egypt, because the Pharoah who did not know Yosef (Joseph) had not yet come to power.
So let us say that 60 out of the 70 from Yaakov's family that arrived in Egypt followed in their father's footsteps and within 7 years had multiple wives and 11 sons and an unknown amount of daughters. So after 25 years in Egypt the 60 would produce at least 660 sons. The recommended marriage age according to Pikei Avot is 18 so I chose to focus after 25 year in Egypt to allow the youngest of the 60 men under discussion to have 7 full years of marriage.
After 50 years in Egypt those 660 sons would have produced at least 7260 sons (11*660). After 75 years the 7260 sons would have produced 79860 (7260*11). From that point in time, even if the amount of sons just doubled every 25 years (since conditions were no longer so favorable. We would have after 100 years in Egypt 159,720 sons (79860*2). At year 125 in Egypt (159720*2) 319,440 sons. At year 150 in Egypt we would have (319440 *2) 638,880. At year 175 in Egypt, we would have 1,277,760. At year 200, we would have 2,555,520 sons.
Now if you take into account the women in the population, who were probably more numerous than the men, because the Egyptians were actively trying to kill some of the men and not the women and you take into account those children born between year 200 in Egypt and the Exodus, as well as the fact that some of the grandfathers and grandmothers were still alive you could have 80 percent of the population dying and still remain with 600,000 men over age 20, leaving Egypt (even without relying on the midrash that the average woman in Egypt gave birth to 6 children at one time).
There are also other scenarios that could lead to having a population 5 times the size of the population that actually left Egypt (in existence slightly before the plague of darkness ) but this scenario is sufficient for me to make the point.
I will also add that it probably is really fitting to have at least a 12 son average and not just an 11 son average (at least during the good times in Egypt) because by Yishmael, he was blessed with having very much children (the Hebrew for very much being מאד מאד) see Breishit Genesis 17:20 where the verse goes on to explain that this means 12 (male) princes.
And in the first chapter of Shmot (Exodus) the increase of Israel is described as very much מאד, מאד
which at the very least should then equal 12 and probably more, since additional terms of increase are used to describe the population growth of Israel (at least during the good times).