Calculus Rate of Change...need help.

The demand for apples is given by the equation xp = 200 , where x is the number of pounds demanded and p is the price per pound. If the price is increasing at a rate of 1 dollars per week, when the demand is 10 pounds, at what rate is the

(a) The rate at which the demand is changing is
(b) The rate at which the revenue is changing is

x * p = 200
x * dp/dt + p * dx/dt = 0

First, solve for p when x = 10

10 * p = 200
p = 20

x * dp/dt + p * dx/dt = 0

x = 10
p = 20
dp/dt = +1
dx/dt = ?

10 * 1 + 20 * dx/dt = 0
20 * dx/dt = -10
dx/dt = -1/2

The demand rate is changing at -1/2 pounds per week
The revenue is never changing