Volume discount

Suppose that the variable cost for using Machine $6$ drops to $$20$ per brake shoe if the company produces more than $150$ total brake shoes on that machine. In either case $($whether they produce at most $150$ or more than $150),$ all brake shoes produced on Machine $6$ will incur the same variable cost.

a$.$ Formulate an IP model for this new problem $($i$.$e$.,$ provide the mathematical model formulation$)$

b$.$ Create a spreadsheet model for this new problem on a new worksheet in the same workbook, and solve it using Solver

c$.$ What is the optimal solution now?