First, you need to find the Unit Product Cost (UPC) for both Absorption Costing (AC) and Variable Costing (VC).
UPC for AC
= Variable Manufactured cost + (Fixed Manufactured cost / Production)
= $100 + ($20 000 / 400 units)
= $100 + $50
UPC for VC
= Variable Manufactured cost
Second, you forgot to put Beginning Inventory (BI) for both AC and VC.
BI for AC
= 150 units x UPC for AC
= 150 units x $150
= $22 500
BI for VC
= 150 units x UPC for VC
= 150 units x $100
= $15 000
Third, you put the wrong value of reconcile income.
(Difference in income = Change in inventory units x Fixed overhead rate per unit)
Fixed overhead rate per unit should be $20,000/400
Here's the summary.
Absorption costing; Net Profit = $25 200
Variable costing; Net Profit = $31 200
Profit difference = $6 000
Reconcile the difference in income for AC and VC
= Change in inventory units x Fixed overhead rate per unit
= 120 x (20000/400)
= 120 x 50
= $6 000