Question

if a 3 - foot yardstick casts a 21 - foot shadow, how tall is a building whose shadow is 168 feet? the building is feet tall.

Explanation:

Step1: Convert yard - stick length to feet

Since 1 yard = 3 feet, a 3 - foot yard - stick.

Step2: Set up proportion

Let the height of the building be $h$. The proportion is $\frac{height\ of\ yard - stick}{length\ of\ yard - stick's\ shadow}=\frac{height\ of\ building}{length\ of\ building's\ shadow}$, so $\frac{3}{21}=\frac{h}{168}$.

Step3: Cross - multiply and solve for $h$

Cross - multiplying gives $21h = 3\times168$. Then $h=\frac{3\times168}{21}$.
$3\times168 = 504$, and $\frac{504}{21}=24$.

Answer:

24 feet