200
How many Jews were living in Egypt prior to the onset of the plagues
What is 15 million.
Here's the math: 600,000 Jewish men between ages 20-60 left Egypt at the Exodus.
Add all the younger and older males for a total of 1.5 million.
Adding the females doubles the total to 3 million.
Rashi (Exodus 13:18, citing the Midrash) says that only 20% of the Jews made it out of Egypt. If 3 million represents 20% of the Jews, then the total number of Jews living in Egypt, prior to the onset of the plagues, was 15 million.