Answer:
a. $ 90,000 cost decrease
Step-by-step explanation:
The computation in the change in the amount of differential cost is shown below:
= (Unit cost by ignoring the fixed cost) - (unit cost to manufacturing the purchase cost) × number of units purchased
= ($12 - $15) × 30,000 units
= $3 × 30,000 units
= $90,000 decrease
And the other information which is given in the question is not relevant. Hence, ignored it