Production Variable

There is no such term as 'production variable'. I can only assume that either you mean variable costs of production, or you have seen this in a question in which case it depends on the context (and will be normal use of English). If it is a past exam question, or a question in the BPP Revision Kit, then tell me which question and I will explain. There is certainly no formula - very few formula are required in Paper PM (and they are given on the formula sheet). Only 50% of the questions involve calculations. I assume that you are watching my free lectures? The lectures are a complete free course for Paper PM and cover everything needed to be able to pass the exam well.