EXPLANATION
Let's see the facts:
Shadow = 30 feet long
height of the building = unknown
flagpole height = 15 feet
flagpole shadow = 10 feet
We can set up a proportion comparing the height of each object to the length of the shadow.
[tex]\frac{h}{s}[/tex][tex]\frac{h}{30}=\frac{15}{10}[/tex]Multiplying both sides by 15:
[tex]h=30\cdot\frac{15}{10}[/tex]Multiplying numbers:
[tex]h=\frac{450}{10}=45_{\text{ }}\text{feet}[/tex]The height is 45 feet