Question

A biologist studies the population of rats in a barn. He found that the average natality was $250$, average mortality $240$, immigration $20$ and emigration $30$. The net increase in population is
A. Zero
B. $10$
C. $15$
D. $05$

Answer 1

Generally, the population has birth, death, emigration and immigration. Number of individuals born in a particular amount of time is the birth rate. It is given as the number of individuals born per year per $1000$ in a population. Mortality is the number of deaths that occur in a given amount of time. The average mortality is the number of deaths per year per $1000$ in a population.

Complete answer
Option A Zero: The net increase in population is
$natality + immigration - mortality - emigration$
Substituting the values,
$250 + 20 - 240 - 30$, we get $0$.
Option A is correct
Option B $10$: Getting $10$ is wrong according to the formula.
Hence, Option B is not correct
Option C $15$: $15$ is not a possible answer if calculated through the equation.
So, Option C is not correct
Option D $05$: Again $05$is a wrong answer in the calculation.
Option D is correct

Hence, Option A is the correct answer

Note:
Immigration is the entry of individuals added to a country or a particular area, but they are born in another country or region. It contributes to the growth of the population as it has its own fertility. Emigration is a process of leaving the residing country, in order to settle in some other country. It can be caused by forced displacement. Sex imbalance and marital fertility disruption may happen due to emigration. Decline in marriages will also happen.