I would assume that the shadow length is proportional to the height of the flagpole. Hence as the building is 5 times higher than the flagpole, the length of the shadow would be 5 times higher, 25x5= 125 ft.
Since you know the height of the pole and the shadow you can write it as a ratio of 40/25. Knowing that the sun hits the building at the same angle you can set up the equation as 40/25 = 200/x where x is the length of the shadow of the building.
40/25 = 200/x
40 = 5000/x Multiply the 25 to the other side
40x = 5000 Multiply the x to the other side. No more fractions!
