Example 13: Three friends split the cost of ordering pizza, and they each pay $13. Which of the following equations models this situation, if p is the cost of the pizza?
A) p = 13
B) 3p = 13
C) 13 = 3
D = 13

Acertain drug is made from only two ingredients: compound a and compound b. there are 3 millimeters of compound a used for every 4 millimeters of compound b. if a chemist wants to make 693 milliliters of the drug, how many milliliters of compound a are needed?