Marginal n absorption costing

A company uses standard absorption costing. Its fixed overhead absorption rate is $8 per machine hour and each unit of production should take 3 machine hours. Last year there was an opening inventory of finished goods of 4000 units. They produced 30,000 units and sold 25,000 units. The actual profit last year was $526K.

What profit would have been earned under a standard marginal costing system?
(the answer is $406K)

here when calculating y u have not take the 4000 inventory.i think the differences between opening n closing is 1000.as (30000-25000)=5000 closing.then OAR should to multiply with 1000 for getting profit differences.m i wrong? plz explain.i m confused with this.

thank you sir in advance