This question cannot be answered properly without the DOM. It transpires the JAA were thinking of DOM when they wrote BEM, it’s just a bad question. What was intended was available payload = MTOM - TO fuel - DOM
and they meant the sum to read
available payload = 5200 - 600 - 3000 = 1600
We have drawn this to the CAA's attention. They agreed that the wrong weight was given and that it should have read DOM.

They have suggested to the JAA that this question should be amended.

We would expect to see BEM already modified to DOM in UK exams but possibly not yet in other European countries.