Physics
posted by Y0gi .
A weather ballon is designed to expand to a maximum radius of 20 m at its working altitude, where the air pressure is 0.030 atm and the temperature is 200 K. If the ballon is filled at atmospheric pressure and 300 K, what is its radius at liftoff?

PV/T is constant. That means PR^3/T is also constant. T must be in Kelvin. If subscript "1" denotes sea level and "2" denotes the working altitude,
(0.03 atm)*(20m)^3/200K = (1.0 atm)*(R1^3)*/300K
Solve for R1 
Oh ok sweetness! Thanks. Figured out the answer. That makes sence. The only thing I had seen wrong with the equation you put is that you would need to find the volume, which is
(4/3)ðr^3 not jut r^3. But still helped out so thanks =) 
They didn't ask for the volume. Anyway, I'm glad you got the right answer.