Toowoomba Solar Limited (TSL) wants to find an equation to estimate some of their monthly operating costs for the coming year. The following cost and other data were gathered for the previous 12 months:

Month	Maintenance costs	Machine-hours	Health insurance	Number of employees	Shipping costs	Units shipped
January	$2,250	1656	$4,300	68	$12,888	1432
February	$2,226	120	$4,300	75	$14,832	1648
March	$2,300	230	$4,300	92	$14,337	1593
April	$2,425	318	$4,300	105	$11,529	1281
May	$2,583	460	$4,300	89	$10,647	1183
June	$2,380	280	$4,300	87	$16,641	1849
July	$2,455	340	$4,300	93	$15,714	1746
August	$2,480	360	$4,300	88	$15,147	1683
September	$2,535	420	$4,300	95	$12,555	1395
October	$2,625	495	$4,300	102	$12,933	1437
November	$2,655	510	$4,300	97	$10,062	1118
December	$2,380	275	$4,300	94	$17,298	1922

a. Which of the above costs is variable? Fixed? Mixed? Explain. (3 marks)

b. Using the highlow method, determine the cost function for mixed cost. (5 marks)

c. Combine the information above to get a monthly utility cost function for TSL. (4 marks)

d. Next month, TSL expects to use 400 machine-hours, have 80 employees and ship 2,000 units. Estimate total operating cost for the month. (3 marks)