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 population is

A population has birth rates and death rates. The rates are expressed as change in numbers (increase or decrease) with respect to members of the population.
In this case, the net increases in population will be Birth rate $(B)+$ Immigration $([)$ - Death rate $(D)$ + Emmigration (l)