At time t, t is greater than or equal to 0, the volume of a sphere is increasing at a rate proportional to the reciprocal of its radius. At t=0, the radius of the sphere is 1 and at t=15 he radius is 2 (the volume V of a sphere with radius r is V=4/3 pi*r^3

a) Find the radius of this sphere as a function of t.

b) At what time t will the volume of the sphere be 27 times its volume at t=0?

I'm completely lost and don't even know where to start. i've been looking at this for about 15 and really need some help. Any pointers would be great