The cost, c, of a ham sandwich at a deli varies directly with the number of sandwiches, n. if c=$54 when n is 9, what is the cost of the sandwiches when n is 3

Since the variation is direct, then we have a function of the form:
 
 From here, we must find the value of the constant k.
 For this, we use the following data:
 c = $ 54 when n is 9
 Substituting values we have:
 
 From here, we clear the value of k:
 
 Therefore, the function is:
 
 For n = 3 we have:
 
 
 the cost of the sandwiches when n is 3 is:
 c = 18 $