A company is increasing production at the rate of 25 units per day. The daily demand function is determined by the fact that the price (in dollars) is a linear function of q. At a price of $70 the demand is 0, and 100 items will be demanded at a price of $60. Find the rate of change of revenue with respect to time(in days)when the daily production(and sales) is 20 items.

how do i approach this? Thanks