Steve, this is the solution:
A. As you can see in the graph, the blue line of the Business Plan is lower at 18 HCF. The difference is $ 4.oo
B. As you can see in the graph, at 14 HCF the cost is the same (both lines are in the coordinate 14, 28).
Again, if the water usage is more than 14 HCF, the Business Plan costs less.