Answer:
252 ft
Explanation:
The object and its shadow are proportional in length, so we can write ...
... b/140 = 18/10 . . . . where b represents the building height
... b = 140·18/10 = 252 . . . . multiply by 140
____
The proportion can be written 3 other ways. They all give the same result.
... b/18 = 140/10
... 140/b = 10/18
... 18/b = 10/140
The idea is that corresponding parts of the triangles are in proportion. The parts of interest can either be two sides of one triangle, or corresponding sides of the two triangles.