Final answer:
The proportion used to find the cost of 9 pints of ice cream, when 3 pints cost $7, is 3 pints / $7 = 9 pints / c. Cross multiplying and solving for c, we find that c = $21, indicating 9 pints cost $21.
Step-by-step explanation:
The question asks us to find the proportion that can be used to calculate the cost, c, for 9 pints of ice cream, given that the cost of 3 pints is 7 dollars. To do this, we set up a proportion where both ratios compare pints to dollars. We know that 3 pints cost $7, so the ratio is 3 pints to $7. To find the cost of 9 pints, we set up the proportion with an unknown value, c, which represents the cost of 9 pints.
The proportion will therefore be:
3 pints / $7 = 9 pints / c
Next, we cross multiply in order to solve for c:
3 * c = 9 * $7
3c = $63
Now, we divide both sides by 3 to solve for c:
c = $63 / 3
c = $21
Therefore, the cost of 9 pints of ice cream is $21.