In the first half of the year, the company constructed 3/5*2000 = <<3/5*2000=1200>>1200 units.
The number of units remaining to be built after the company built 1200 units in the first half of the year is 2000-1200 = <<2000-1200=800>>800
By October, the company built an additional 300 units, thus remaining with 800-300 = <<800-300=500>>500 units to be built.
#### 500