A biologist studied 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 populations is :

Options

A. 5
B. zero
C. 10
D. 15

Answer

To find the net increase in the population of rats in the barn, we need to consider the factors that contribute to population growth and decline. The relevant factors are natality (birth rate), mortality (death rate), immigration (individuals moving into the population), and emigration (individuals moving out of the population).

### Step-by-Step Solution:

1. **Identify the given values:**
- Natality (birth rate) = 250
- Mortality (death rate) = 240
- Immigration = 20
- Emigration = 30

2. **Calculate the total increase in population:**
- Total increase = Natality + Immigration
- Total increase = 250 + 20 = 270

3. **Calculate the total decrease in population:**
- Total decrease = Mortality + Emigration
- Total decrease = 240 + 30 = 270

4. **Calculate the net increase in population:**
- Net increase = Total increase - Total decrease
- Net increase = 270 - 270 = 0

### Conclusion:
The net increase in the population of rats in the barn is **0**.