Need help with total contribution margin on lost sales (18, is not the correct answer)

Hrubec Products, Inc., operates a Pulp Division that manufactures wood pulp for use in the production of various paper goods. Revenue and costs associated with a ton of pulp follow: Hrubec Products has just acquired a small company that manufactures paper cartons. This company will be treated as a division of Hrubec with full profit responsibility. The newly formed Carton Division is currently purchasing 31, 000 tons of pulp per year from a supplier at a cost of $20 per ton, less a 10% purchase discount. Hrubec's president is anxious for the Carton Division to begin purchasing its pulp from the Pulp Division if an acceptable transfer price can be worked out. For (1) and (2) below, assume that the Pulp Division can sell all of its pulp to outside customers for $20 per ton. What is the minimum Transfer Price at which Pulp is willing to sell within the firm