Solve by calculating related rates

1. What is the radius of an expanding circle at a moment when the rate of change of its area is numerically twice as large as the rate of change of its radius?

2. If the radius of a sphere is increasing at the constant of 3 millimeters per second, how fast is the volume changing when the surface area 4r² is 10 square millimeters?

3. A 15 foot ladder is resting against the wall. The bottom is initially 10 feet away from the wall and is being pushed towards the wall at a rate of 1/4 ft/sec. How fast is the top of the ladder moving up the wall 12 seconds after we start pushing?