Need 9n inches of ribbon
Cost is $0.75n
Since you need 9 inches per party favor and you have n party favors, the amount of ribbon you need is 9 times n inches. So
R = 9n inches
Since the ribbon costs $3 per yard and our current equation is inches, we need to do a bit of conversion. There's 36 inches per yard and 9 inches per ribbon, so each yard of ribbon can make 36/9 = 4 ribbons for the party favors. So the cost per party favor will be one forth of the cost per yard of ribbon per party favor. So $3.00 / 4 = $0.75, making the cost equation:
C = $0.75n
