Answer:
Part 1.  
Plantwide overhead rate for Laval using direct labor hours as a base. is $1.60 per Direct Labor Hour
Part 2. 
Total manufacturing cost per unit for the aluminum desk lamp using the plantwide overhead rate is $78.76
Part 3. Compute departmental overhead rates based on machine hours in the fabricating department and direct labor hours in the assembly department.
                                          Fabricating                  Assembly              
Overheads (R)                      390000                         410000       
Department Cost Driver      152000                         290000       
Overhead Rate                         2.57                                 1.41            
Therefore Overhead Rates are :
             Fabricating Department $ 2.57 per Machine Hour  
             Assembly Department $1.41 per Labor Hour           
Part 4. Use departmental overhead rates from requirement 3 to determine the total manufacturing cost per unit for the aluminum desk lamps.
Direct materials ($270000/21000)                                         12.86
Direct labor:
        Fabricating department(6500/21000×$29)                   8.98
        Assembly department(15200/21000×$26)                   18.82
Overheads:
        Fabricating department(152000/21000×$2.57)           18.60
        Assembly department (290000/21000×$1.41)             19.47
Total manufacturing cost per unit                                         78.73
Explanation:
Part 1.  Plantwide overhead rate for Laval using direct labor hours as a base. 
Overhead Rate = Total Overheads/Total Direct Labor Hours
                           = $1.60 per Direct Labor Hour
                                             Fabricating                  Assembly         Total      
Overheads (R)                      390000                         410000       800000
Direct Labor Hrs                  210000                         290000       500000
Overhead Rate                                                                                   1.60
Part 2. Total manufacturing cost per unit for the aluminum desk lamp using the plantwide overhead rate 
Direct materials ($270000/21000)                                         12.86
Direct labor:
        Fabricating department(6500/21000×$29)                   8.98
        Assembly department(15200/21000×$26)                   18.82
Overheads:
        Fabricating department(210000/21000×$1.60)            16.00
        Assembly department (290000/21000×$1.60)            22.10
Total manufacturing cost per unit                                         78.76
Part 3. Compute departmental overhead rates based on machine hours in the fabricating department and direct labor hours in the assembly department.
Part 4. Use departmental overhead rates from requirement 3 to determine the total manufacturing cost per unit for the aluminum desk lamps.