I doubt very much that Steve Scott is going to set a question with closing stock of intra-group goods which do not have a profit element within them
If there are goods in inventory purchased from another group company, then do your pup calculation to find the profit element, apply the profit element fraction to the affected closing inventory value , and there’s your pup.
If there is no affected inventory then, when you apply the profit element fraction, your answer would look like ( say ) 20/120 * zero = zero pup
No, I don’t think you’re likely to face that scenario