Answer:
Agon
Allocated joint cost $45
Sales value split off $90
additional processing cost $60
Sales value if processed further $150
Contribution from processing further $15
Xon
Allocated joint cost $75
Sales value split off $150
additional processing cost $150
Sales value if processed further $300
Contribution from processing further $0
Zeon
Allocated joint cost $330
Sales value split off $660
additional processing cost $180
Sales value if processed further $1200
Contribution from processing further $360
Explanation:
Sales split off point Xon $300 - $150 = $150
Sales split off point Zeon $900 - $90 - $150 = $660
Joint cost of Xon = 450 / 900 * 150 = 75
Joint cost of Zeon = 450 / 900 * 660 = 330