Question: After 100 new people move into a town, 400 of the original population move out. Every year afterward, the town's population is halved. After 4 years, the population is 60 people. Before the new people moved in, how many people lived in the town? Think carefully first, then make a decision:
If the population is halving then at the end of the third year, the population must have been 60 people in the fourth year * 2 = 120 people. In the second year, the population was 120 people in the third year * 2 = 240 people. And in the first year, the population was 240 people in the third year * 2 = 480 people. Additional people moved out in the first year, so accounting for these shows the population was 480 people + 400 people who moved out = 880 people. Then removing the new people shows there were originally 880 people 100 new people = 780 people. So the answer is 780.