A child loses his balloon, which rises slowly into the sky. If the balloon is 13 cm in diameter when the child loses it, what is its diameter at the following altitudes? Assume that the balloon is very flexible and so surface tension can be neglected.
(a) 2200 m
They should have told you whether to assume the temperature is the same at the two altitudes. In a standard atmosphere, it won't be. Also, the surface tension of a child's balloon is not negligible.
My suggestion: Assume T = constnat, use a standard atmosphere for the pressure change P2/P1, and use
P*V = constant