Answer:
6 cups
Explanation:
When two things are directly proportional (e.g. y and x), we can model the relationship with the direct variation equation, which is
y = kx, where
- k is the constant of proportionality
Step 1: To find k, we can allow y to represent Jackie's total revenue from selling lemonade ($9) and x to represent the quantity of cups of lemonade sold ($3):
9 = 3k
k = 3
Step 2: Now that we've found the constant of proportionality, we can plug in 18 for y and 3 for k, allowing us to solve for x:
18 = 3x
6 = x
Thus, in order for Jackie to earn $18, she must sell 6 cups in all.