Answer:
differential cost increase = $60,000
Explanation:
Frank currently pays $25 per unit x 20,000 units = $500,000
if it can produce the parts itself, the differential cost will be only the variable costs = $30 - $2 (fixed costs) = $28 per unit x 20,000 units = $560,000
It would be a bad decision for Frank to produce the parts because they would spend $60,000 more than if they continue to purchase them from a vendor.