11. Monthly sales revenue, S (in $), and monthly advertising expenditure, A (in $), are modelled by the linear relation, S = 9000 + 12A.
(a) If the firm does not spend any money on advertising, what is the expected sales revenue that month?
(b) If the firm spends $800 on advertising one month, what is the expected sales revenue?
(c) How much does the firm need to spend on advertising to achieve monthly sales revenue of $15 000?
(d) If the firm increases monthly expenditure on advertising by $1, what is the corres- ponding increase in sales revenue?

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