Answer:
 .
.
Explanation:
Let x represent the number of cupcakes and y represent total cost.
We know that two proportional quantities are in form 
 , where, k represents constant of proportionality.
, where, k represents constant of proportionality.
1 dozen = 12 
Half dozen = 6
Let us substitute 
 and
 and 
 in above equation.
 in above equation.




Therefore, the constant of proportionality is 2.5 and our required equation would be 
 .
.