Answer:
20 feet
Step-by-step explanation:
Since they are two right triangles, you can prove that triangle BAD is congruent with triangle CAD through HL (hypotenuse leg)
BA = AC and AD = DA (with the reflexive formula)
Since they both have right angles, HL would be what proves that the two small triangles are congruent
Since BD = 20 feet then DC should also equal 20 feet since they are congruent triangles
