You measure the shadow cast by a certain tall building and find it to be 100 feet long. then you measure the shadow cast by a three-foot post and find it to be4 inches long. determine the building's height.

First, set up a ratio:
The 1,200 in the first fraction is the length of the shadow in inches, and is the height of the building. The 4 is the length of the pole's shadow, and the 36 is it's height (both figures also in inches). Now divide the pole's height by it's shadow, and multiply that number by the building's shadow length:
This answer is in inches, so, divide the building's height by twelve to get the answer in feet: feet