Final answer:
Peterson's finished goods inventory cost under the variable costing method is $90,000, determined by the variable cost per unit multiplied by the remaining units in inventory at the year-end.
Step-by-step explanation:
To calculate Peterson's finished goods inventory cost at December 31 under the variable costing method, we need to consider only the variable costs associated with the production of the goods. Since no work was in process at the beginning or end of the year, we can assume all costs were for goods either sold or in finished goods inventory. The total variable manufacturing costs were $630,000 for 70,000 units manufactured. Since each unit cost the same to produce, the cost per unit is $630,000 / 70,000 units = $9 per unit. Peterson sold 60,000 units, leaving us with 70,000 - 60,000 = 10,000 units in inventory. Therefore, the finished goods inventory cost is 10,000 units × $9/unit = $90,000.