I do not have the BPP workbook, only the Revision Kit.
Without seeing the question I am afraid that I cannot help you.
Have you watched my free lectures on process costing? The lectures are a complete free course for Paper MA and cover everything needed to be able to pass the exam well. If you are watching the lectures then you do not really need the workbook. (The essential book, however you choose to study, is the Revision Kit. It is full of exam standard questions for practice, and practice is vital to passing the exam.)