Pen Pro produces three main lines of felt tip pens $-$ Prime, Deluxe and Regular models. Pen Pro received a forecast to sell $30,000$ pens $($combined for all three lines$)$ for the coming month from its marketing department. Fixed costs of $$45,000$ per month are allocated to felt$-$tip operations. The variable costs and selling price of the three models are shown on the table below. Also shown are the historical percent market shares of each of the product lines.

a$.$ Determine the total charges under each plan for this case: $300$ minutes of day calls and $300$ minutes of evening calls in a month.

b$.$ Prepare a graph that shows total monthly cost for each plan for daytime call minutes only.

c$.$ If the agent will use the service for daytime calls, over what range of call minutes will each plan be optimal $($i$.$e$.$ least costly$)?$

d$.$ Suppose that the agent expects both daytime and evening calls. At what point $-$ percentage of call minutes for daytime calls $-$ would she be indifferent between plans A and B $($i$.$e$.$ the total costs are equal$)?$