Throughput suggest only direct material cost is variable in nature and all other cost i.e (direct labour ,variable overheads and other fixed overheads) are consider fixed costs throughput/unit = S.P – direct material/unit thats why the answer takes 144,000 units x $28 for product A and 15,000 units x $4 for direct material calculations because its variable i-e they are changing with the level of activity as throughput implies. All other costs are fixed therefore 120,000units for product A and 45,000units for product B are being used because they do not change with the level of activity.