Boyle's law for the expansion of gas is PV = C, where P is the number of pounds per square unit of pressure, V is the number of cubic units of volume of the gas, and C is a constant. At a certain instant the pressure is 3000 pounds per square feet, the volume is 5 cubic feet, and the volume is increasing at the rate of 3 cubic feet per minute. Find the rate of change of the pressure at this instant.