If a 36-inch yardstick casts a 21-foot shadow, how tall is a building whose shadow is 168 feet?

The sun must be very low in the sky, as a 24foot building would cast the 168 foot shadow.

Always be careful that the units of your calculation are consistent. It is a common trick in word problems to see if you are attentive. It is best to stay in feet rather than inches in this problem. The 36 inches is equal to 3 ft.

I prefer to just use a proportion to solve: ##3/ 21## = ##x/168## Using cross products we calculate x = 24 ft.