Absorption cost

A company uses a standard absorption costing. Its fixed overhead absorption rate is 8 per machine hour and each unit of production should take 3 hours. Last year there was an opening inventory of finished good of 4000. They produce 30000 and sold 25000. The actual profit last was $526000. What would have been earned under marginal costing. I worked out this question following below rule. Actual profit 526000 Closing inventory 5000 Less Opening inventory 4000 Increasing of inventory 1000 *24 24000 Profit under marginal costing 502000 The mock exam provides 406000 as correct answer that I suppose it does not take into consideration the opening inventory. Could you please help me to understand. Thanks Gabbi