The length of the highway that needs to be constructed is 650 – 200 = <<650-200=450>>450 miles.
After the first day, there are still 450 – 50 = <<450-50=400>>400 miles that need to be constructed.
On the second day, 50 miles * 3 = <<50*3=150>>150 miles are added to the highway.
This means that 400 – 150 = <<400-150=250>>250 miles still need to be added to the highway.
#### 250