# Related Rate -calc

1. Oct 23, 2004

### bjr_jyd15

Hi everyone. I'm stuck on this problem in calc:

A spherical balloon is inflated with gas at the rate of 500 cubic centimeters per minute. How fast is the radius of the balloon increasing at the instant the radius is a)30 cm and b)60 cm?

Here's what I have so far.

V=4/3*pi r^3

dV/dt=500

dr/dt=?

dV/dt=4/3*pi (3r^2) dr/dt

Where can I go from here? Any help would be great. Thanks

2. Oct 23, 2004

### arildno

OOPS, MAJOR EDIT:
"dV/dt=4/3*pi (3r^2) dr/dt"
You've got this equation, right?
So, regrouping you have:
$$\frac{dr}{dt}=\frac{1}{4{\pi}r^{2}}\frac{dV}{dt}$$

Last edited: Oct 23, 2004
3. Oct 24, 2004

### bjr_jyd15

got it

thanks, i got it. The 4/3 just screwed me up. i just plugged dv/dt from original info.