The formula to calculate the cost per stamp is:
\[ \text{CPS} = \frac{\text{CS}}{\text{S}} \]
Where:
The average cost of a first-class stamp is 55 cents. The average cost of a postcard stamp is 35 cents.
Example:
Calculation:
\[ \text{CPS} = \frac{11}{20} = 0.55 \text{ dollars per stamp} \]
