Short Term Decision

Dear Mr Moffat, I did below question but it still not completely clear to me the answer provided. Could you kindly help? A company produces three components on the same machine. The components are used in the manufacture of a finished product. The budget for next year indicates a requirement for 3000 units of each component, but only 60000 of machine time will be available. Additional components and be purchased from an external supplier to meet any production shortfall. Component Machine hour per unit VC per Unt Purchase Price Per unit A 9 45 65 B 5 70 78 C 12 56 80 What is the minimum total variable cost at which the 3000 units of all three components can be obtained? Answer Machine hours Variable Cost Make A 3000 units 27000 135000 Make C (balance 2750) 33000 154000 Buy 250 C at 80 20000 Buy 3000 B at 78 234000 Total 534000 Why has the cost of buying material been included in the calculation of the variable cost? When I did the question I only considered 135000+154000. Thanks Gabbi