When the number of bags is 3, the number of ounces is 36:
36 = k * 3
k = 36 : 3
k = 12
Also, when the number of bags is 4, the number of ounces is 48:
48 = k * 4
k = 48 : 4
k = 12
Finally, when the number of bags is 5, the number of ounces is 60:
60 = k * 5
k = 60 : 5
k = 12
Answer: The constant of proportionality is k = 12.
