Answer:
1. Variable cost = Shipping costs
Fixed cost = Health Insurance
Mixed cost = Maintenance costs
Shipping costs are variable because a unit shipped costs $3.60. The total shipping cost for each month varies according to the units shipped in the month.
Health Insurance costs are fixed as there is no change in cost notwithstanding the number of employees in each month.  The total health insurance cost remains the same every month.
Maintenance costs are mixed for each month, as there is a fixed element and a variable element.
2. Cost function for each cost:
Maintenance = $4,200 + $2.10 per machine hour
Health Insurance = $8,600
Shipping cost = $3.60 per unit
3. Cost function = $12,800 + $2.1m + $3.6s
where m = machine hours
and s =  units shipped
4. The total operating cost for the month 
= $46,040
Explanation:
a) Data and Calculations:
Month     Maintenance Machine  Health   Number of  Shipping     Units
                      costs        Hours  Insurance  Employees   Costs    Shipped
January          4500       165         8600            68            25778       7160
February        4452       120         8600            75           29664      8240
March            4600       230        8600            92            28674      7965
April               4850        318        8600           105           23058      8405
May                5166       460        8600            89            21294       5915
June              4760       280        8600            87            33282      9245
July                4910       340        8600            93             31428      8730
August         4960       360        8600            88            30924       8415
September  5070       420         8600            95             25110     6975
October      5250       495         8600           102           25866      7185
November   5271        510         8600            97             20124    5590
December  4760       275         8600            94            34596     9610
Cost Function for each cost:
Maintenance cost:
                      Machine Cost
                         Hours
November         510    5271 
February           120    4452 
Difference       390      819
Variable cost = $2.10 (819/390) 
Fixed cost = $4,200 ($5,271 - ($2.10*510))  
Health Insurance:
Fixed cost = $8,600
Shipping cost:
Variable cost = $3.60 per unit
Cost function = $4,200 + $2.10m + $8,600 + $3.60s
= $12,800 + $2.1m + $3.6s
February cost = $12,800 + $2.1(120) + $3.6(8240)
= $12,800 + $252 + $29,664
= $42,716
IF:
Machine hours = 400
Employees = 80
Shipped units = 9,000
The total operating cost for the month will be:
Cost function = $12,800 + $2.1m + $3.6s
= $12,800 + ($2.1 * 400) + ($3.6 * 9,000)
= $12,800 + $840 + $32,400
= $46,040