Diagram shows a section of a bridge between the points a and k. the length of line segment is 640 meters. ∆abc, ∆cdf, and ∆fjk are similar, and 2ac = cf = 2fk. the first pillar, , is 20 meters tall.
the area of ∆cdf is square meters.

AK = 640
Δ ABC and ΔFJK are similar. They are small triangles.
ΔCDF is the big triangle.

640 / 2 = 320 m= CF
AC & FK= 320/2 =  160 m each

2AC = CF = 2FK
2(160) = 320 = 2(160)

BG = 20 m

20/160 = x / 320
20*320 = 160x
6400 = 160x
6400/160 = x
40 = x

Area of CDF = (40 m * 320 m)/2 = 12,800 / 2= 6,400 m²

In this problem it decreased 20%