A rock is thrown into a still pond. The circular ripples move outward from the point of impact of the rock so that the radius of the circle formed by a ripple increases at the rate of 5 feet per minute. Find the rate at which the area is changing at the instant the radius is 15 feet. When the radius is 15 feet, the area is changing at approximately square feet per minute. (Round to the nearest thousandth as needed.)