Transfer Price A

Hello Sir, Hope you are keeping well amid the Covid situation around the world! Can you please help me the below question: Manuco company has been offered supplies of special ingredient Z at a transfer price of $15 per kg by Helpco company, which is part of the same group of companies. Helpco processes and sells special ingredient Z to customers external to the group at $15 per kg. Helpco bases its transfer price on full cost plus 25% profit mark-up. The full cost has been estimated as 75% variable and 25% fixed. Internal transfers to Manuco would enable $1.50 per kg of variable packing cost to be avoided. My Ques- How is the variable cost of 9 (as per the answer) is calculated ?