You measure the shadow cast by a certain tall building and find that it is 105 feet long. Then you measure the shadow by a three foot post and find that is 4 inches long. Determine the buildings height

4 inches = 1/3 of a foot.

The post is 3 foot, the shadow is 1/3 foot

Find the scale factor:

3 / 1/3 = 3*3/1 = 9.

The post is 9 times longer than the shadow.

This means the building is 9 times bigger than it' shadow:

105 x 9 = 945

The building is 905 feet tall.