Please, proofread your problem statements before posting them. Your "If 1,600 people live 25% of the population how many people need to leave" should be written as "If the whole population is 1,600 and 25% of them need to leave, how many must leave?"
0.25(1600) = 400
400 people need to leave. This is 25% of the entire population, or 25% of 1600 people. Please note: it's "leave," not "live."
