Roadrunner Ltd sells biscuits in boxes to supermarkets throughout the country. In the financial year ended 30 June 2021, the company sold a total of 500,000 boxes of biscuits at $4 per box. The gross profit margin achieved on these sales was 75%. Operating expenses consisted of the following: Advertising Staff salaries Other administrative expenses $ 100,000 500,000 600,000 Cost of sales are considered as variable costs, and all other expenses are treated as fixed costs. Required: a) Calculate the contribution margin per box of biscuits. (1 mark)