He estimates the building on the left is 80 feet tall because 100 x .8 = <<100*.8=80>>80
The combined height of the left and middle is 180 because 100 + 80 = <<100+80=180>>180
The building on the right he estimates as 160 feet because 180 - 20 = <<180-20=160>>160
He estimates the combined height as 340 feet because 80 + 100 + 160 = <<80+100+160=340>>340
#### 340