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 = <<60*2=120>>120 people.
In the second year, the population was 120 people in the third year * 2 = <<120*2=240>>240 people.
And in the first year, the population was 240 people in the third year * 2 = <<240*2=480>>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 = <<480+400=880>>880 people.
Then removing the new people shows there were originally 880 people – 100 new people = <<880-100=780>>780 people.
#### 780