Answer:
The savings from the switch is negative savings of $6300,in other words loss is recorded not savings in costs.
The interest would have to fall to 10% for the planned switch to production to be feasible.
Explanation:
The extra cost savings of switching over to level production is given below
Cost savings                             $42000
interest on inventory finance($420000*11.5%) Â ($48300)
Negative savings                           ($6,300)
The company would a loss of $6300 if it switches to level production,hence it is advisable to shelve the plan for now.
However,interest rates would have to fall to 10% Â as calculated below to make the planned switch to level production feasible
new interest rate=savings/increase in inventory
              =42000/420000*100
              =10%