A biologist studies the population of rats in a barn. He found that the average mortality was 240, average natality 250, immigration 20 and emigration 30. The net increase in population is?

Net increase in population can be calculated by a formula: (B+I) - (D+E) where in B refers to Birth rate/Natality, I refers to Immigration, D refers to Death rate/Mortality and E refers to Emigration. Therefore net increase for the population of rats in barn is (250+20) - (240+30), thereby 270-270. Hence net increase is zero and option C is correct answer.