53. There are n arithmetic means between 40 and 144 such that 4th mean :nth mean is 2:5. The common difference is (a) 2
(b) 3
(c) 4
(d) 5​

Suppose that the price p (in dollars) and the weekly sales x (in thousands of units) of a certain commodity satisfy the demand equation 4p cubedplusx squaredequals38 comma 400. determine the rate at which sales are changing at a time when xequals80, pequals20, and the price is falling at the rate of $.20 per week.