A certain man works in sales and earns a base salary of $1200 per month plus 4% of his total sales for the month. (a) Explain why his total monthly income I is a linear function of total sales S, both measured in dollars. The change in I is always the same, dollar , for a dollar increase in S. (b) How much does he eam if he sells $1400 in merchandise in a month? (Round your answer to the nearest cent.) $ (c) Write a formula that gives total monthly income I as a linear function of sales S in a month. (Round equation parameters to tivo decimal places.) I= (d) What should his monthly total sales be if he wishes to earn $1400 this month? (Round your answer to the nearest cent.) $