∴x？ ( 100-4x)=400

100x - 4x？ - 400=0

-4(x？ - 25x + 100)=0

(x-5)(x-20)=0

∴x=5 or x=20

When AB=5, BC= 100-4? five

= 100-20 = 80m, and the known middle wall is only 25m, so it is abandoned.

∴ AB = 20m

So BC= 100-4? 20 = 100-80 = 20m