Answer:
The constant of proportionality is 2.5 dollar per cupcake and then the required equation would be 
 .
.
Explanation:
Given:
Let the number of cupcakes be represent by 

Also let the total cost be represented by 

We know that two proportional quantities are in for;

where, k⇒ represents constant of proportionality.
Now we know that;
1 dozen = 12
Half dozen = 6
Now Given:
half dozen cupcakes cost $15.
So Let us substitute 
 and
 and 
 in above equation we get;
 in above equation we get;

Dividing both side by 6 we get;

Hence the constant of proportionality is 2.5 dollar per cupcake and then the required equation would be 
 .
.