Math problems, asking for help

Drawing, cd is grounded at point C, ab is grounded at point B, and Xiao Zhang is grounded at point F for ef.

Connect cbf in a straight line, connect dae in a straight line, connect cd in a straight line and connect ab in a straight line; Cbf and dae meet at point k,

Make a straight line ag parallel to cbf and intersect cd at G point; Make a straight line eh parallel to cbf, and intersect with cd at point H;

Then DG = DC-CG =10 (CG = AB = 20);

DH = DC-HC = DC-ef = 30- 1.6 = 28.4(ef = HC = 1.6)；

So according to similar triangles, dg/ga=dh/he, (GA = BC = 30);

he = ga * DH/DG = 30 * 28.4/ 10 = 85.2

Cf=he=85.2 (m),

Cf is the nearest distance from the teaching building, and you can see the banner, which is 85.2 meters.