Evaluating Formulas by Substitution Examples

Sara has a clock with a circular face. She wants to glue a ribbon around the outer edge of the face of the clock. 

Heaven knows why she thinks this will look good. It would be enough to make Martha Stewart have a conniption fit. Ah, well. To each her own.

If the face of the clock has a radius of 5 inches, how much ribbon will Sara need to complete her aesthetic disaster?

The formula for the circumference of a circle is C = 2πr. Evaluate for r = 5 to find that C = 10π. Sara will need 10π, or approximately 31.4, inches of ribbon. Hey, Sara—guess what time it is? Time to call a professional decorator.