Final answer:
The birthrate is calculated by dividing the number of births (400) by the initial population size (1000) and expressing it per 1,000 individuals, which results in equation 2 being the correct one to use for this calculation.
Step-by-step explanation:
To calculate the birthrate of a population, where the population initially has 1000 individuals and 400 new individuals are born over a year, we would use the number of births divided by the initial population size and often express it per 1,000 individuals to standardize the measurement. So, the second equation provided, "400 ÷ 1000 = 0.4" computes the per capita birth rate (proportion of the population giving birth). To express it per 1,000 individuals, we would then multiply the per capita birth rate by 1,000. This would give us a birthrate of 400 per 1,000 per year. The correct birthrate calculation is option 2: 400 ÷ 1000 = 0.4.