Can be used to solve  problems with ratios.


Problem: The shadow of a flagpole is 70 ft and the shadow of your mom is 10 ft. If your mom is 5 ft 3 inches, how tall is the flag pole?

Ratio shadow length: height

  • your mom → 10 ft : 5.25 ft
  • flagpole → 70 ft : x ft

Change ratios in proportions

\frac{height}{shadow}=\frac{5.25\ ft}{10\ ft}=\frac{x\ ft}{70\ ft}

* cross multiplication


x=\frac{5.25(70)}{10}=36.75\ ft


Scale room problem: A scale drawing is made with the length of the room as 12 cm and width as 7 cm. If the actual length of the room is 30 ft, then what is the actual width?

\frac{length}{width}=\frac{12\ cm}{7\ cm}=\frac{30\ ft}{x\ ft}

* cross multiplication



x=17.5\ ft