With a big assist from GoldaLeah, the WesternJew:
Part I In which the question is asked.
Part II In which her Rabbi hems and haws around the obvious answer.
The verse:
Rashi (Ex. 10: 21-23) says that 80 percent of the Jewish people (about 12 million souls) died during the plauge of darkness.
The questions:
(1) Why doesn't the Torah mention this?
(2) How can the exodus be thought of as a time of great celebration if everyone was in deep mourning?
(3) How is it possible that fifty-five men who went down to Egypt with Yaakov could be the ancestors, over 210 years, of 15 million people? (Ibn Ezra)
(4) During the 10 plagues, the Jews were always spared. How is it possible that Jews died -and in such great numbers - during a plauge that killed no Egyptians? (Ibn Ezra)
The obvious answer
"The whole thing is a derash; don't rely on it. Maybe the one who said it at the outset had a hidden reason (sod)." --Ibn Ezra