The cost per sandwich when 3 are delivered is $20/3 or $6.67 each.
The cost per sandwich when 4 are delivered is $26/4 or $6.50 each.
The average cost per sandwich changes with the number delivered, so it is not proportional. The cost decreases per sandwich as the volume of sandwiches increases. One can't predict the cost of 6 delivered sandwiches based on the information provided. If it were proportional, the cost could be calculated on the basis of the slope of the line. If all sandwiches cost $6.50 each, regardless of the volume ordered, the slope of a line relating number ordered and total cost would have a constant slope of 6.50. That isn't the case in this problem. The slope keeps changing.
