Question 253033
There is a guide for problem solving called Occam's Razor. The general concept is that among competing hypotheses, the method requiring the fewest set of assumptions is usually the best way to solve the problem. There are several ways to solve the problem of establishing the height of the building. In addition to an algebraic problem it can also be considered using Occam's Razor as a thought problem. This we know:
1. The shadow of the building is 280 feet long.
2. The shadow of a 5 foot boy (standing nearby) is 2 feet long.

So how high is the building? It is easiest to break down numbers into their basic units; here we are dealing with a basic unit of 1 foot. So let's shrink the boy down so that his shadow is 1 foot long. That means the boy is 2 1/2 feet tall. If a shadow of 1 foot results in a height of 2 1/2 feet, then a shadow of 280 feet results in a height of X feet. So 2 1/2 times 280 is.... 

Search the WEB for Occam's Razor. You will find the concept helpful with future problems.