17.3 ft
1) In this problem Since that height is always perpendicular, we have here a right triangle formed by the ladder to the wall
2) We have the hypotenuse and one leg. Let's calculate the missing leg.
a² =b² +c ²
hypotenuse² = leg² +leg²
20² =10² +c²
400 = 100 + c²
400 - 100 = c²
300 = c²
√c² =√300
c =17.32 ≈ 17.3 ft.
3) Hence the height of that building is approximately 17.3 ft