To find the population size at the beginning of the first year, we can use the formula:
Population at the beginning of the first year = (Total population at the end of the second year) / (1 + (percentage increase)) * (1 - (percentage increase))
Plugging in the values from the given information, we get:
Population at the beginning of the first year = 9975 / (1 + (5/100)) * (1 - (5/100))
Simplifying this expression, we get:
Population at the beginning of the first year = 9975 * (20/21) * (20/19)
Which simplifies to:
Population at the beginning of the first year = 10000
So the population at the beginning of the first year was 10000.