Homework Help: Related rates Balloon question. Thanks

1. Nov 5, 2012

nukeman

1. The problem statement, all variables and given/known data

Air is being pumped into a spherical balloon. Suppose we know the surface area of the balloon increases at a rate of 20cm^2/s when its radius is 4cm. What is the rate its volume is changing at that instant?

2. Relevant equations

3. The attempt at a solution

I went....

dv/dt = 4∏ (4)^2 * 20/1 = 1280∏

I know its wrong. Anyone help me out? What am i doing wrong?

Last edited: Nov 5, 2012
2. Nov 5, 2012

nukeman

Wait no, I went:

dv/dt = (1/4pi(4)^2) *20/1

gives me 5 / 16pi

3. Nov 6, 2012

haruspex

I think that's still wrong. Write out generic formulae for dV/dt and dA/dt as functions of r and dr/dt..