rate of revenue help?

Question

A company's revenue from selling x units of an item is given as . If sales are increasing at the rate of 60 units per day, how rapidly is revenue increasing (in dollars per day) when 180 units have been sold?

Answer 1

R = 1400x - 2x²

The problem asks for the rate of change of R in dollars per day, which is the first derivative of R (dollars) wrt to time (days):

dR/dt = dR/dx · dx/dt = (1400 - 4x)·dx/dt

The problem states that x = 180 and dx/dt = 60. Plug those numbers in to compute the answer. Units are dollars/day.